Memory device, and semiconductor device and electronic appliance including the same

ABSTRACT

A memory device capable of optimizing a refresh cycle is provided. The memory device includes a monitor circuit capable of generating a signal serving as a trigger for a refresh operation. The monitor circuit includes a transistor and a capacitor. The monitor circuit has a function of sensing that a potential retained in the capacitor is lower than a reference potential, a function of generating a first signal and a second signal on the basis of the sensing result, and a function of turning on the transistor in response to the second signal and resetting the potential retained in the capacitor to an initialization state. It is possible to start refresh of a memory cell in response to the first signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/723,618, filed May 28, 2015, now U.S. Pat. No. 9,406,370, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2014-111062 on May 29, 2014, both of which are incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a memory deviceincluding a semiconductor or a semiconductor device, a driving methodthereof, a manufacturing method thereof, and the like.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in the specification, the drawings, and the claims(hereinafter referred to as “this specification and the like”) relatesto an object, a method, or a manufacturing method. Furthermore, oneembodiment of the present invention relates to a process, a machine,manufacture, or a composition of matter. Specifically, examples of thetechnical field of one embodiment of the present invention disclosed inthis specification and the like include a semiconductor device, a memorydevice, a display device, a liquid crystal display device, alight-emitting device, a lighting device, a power storage device, aninput device, an imaging device, a method of driving any of them, and amethod of manufacturing any of them.

2. Description of the Related Art

In a general dynamic random access memory (DRAM), a memory cell includesone writing transistor (1T) and one capacitor (1C). Such a 1T1C DRAM isa memory capable of retaining data by accumulating charge in thecapacitor and thus has no limit on the number of times of writing inprinciple. As a high-capacity memory device, the DRAM is incorporated ina number of electronic appliances because of writing and reading atrelatively high speed and a small number of memory cells, which easilyenable high integration. Besides the 1T1C memory cell, a memory cellcalled a gain cell including two or three transistors is known.

Even when a writing transistor is in an off state, a slight amount ofleakage current is generated between a source and a drain; thus, thedata is lost within a relatively short time. Therefore, the data needsto be rewritten (refreshed) on a regular cycle (generally once everyseveral tens of milliseconds) in a DRAM.

The use of a transistor in which the channel is formed using an oxidesemiconductor (hereinafter may be referred to as an OS transistor) in a1T1C memory cell and the use of the OS transistor as a writingtransistor of a gain cell are proposed. For example, Patent Document 1discloses that even when power is not supplied, data can be retained ina memory cell for a long period by utilization of a characteristic of anextremely small off-state current of an OS transistor. Furthermore, acircuit which has a function of detecting the timing of refresh of amemory cell including an OS transistor is proposed (e.g., PatentDocuments 2 and 3).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2011-187950

[Patent Document 2] Japanese Published Patent Application No.2012-064930

[Patent Document 3] Japanese Published Patent Application No.2012-256408

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel semiconductor device and a method for driving the novelsemiconductor device. Another object of one embodiment of the presentinvention is to provide a semiconductor device with improved reliabilityand a method for driving the semiconductor device. Another object of oneembodiment of the present invention is to provide a memory devicecapable of reducing power consumption and a method for driving thememory device. Another object of one embodiment of the present inventionis to provide a memory device capable of optimizing a refresh cycle anda method for driving the memory device. Another object of one embodimentof the present invention is to provide a memory device capable ofperforming temperature compensation or a method for driving the memorydevice.

Note that the description of a plurality of objects does not mutuallypreclude the existence. One embodiment of the present invention does notnecessarily achieve all the objects. Objects other than those listedabove are apparent from the description of the specification, drawings,and claims, and also such objects could be an object of one embodimentof the present invention.

A memory device of one embodiment of the present invention includes amemory cell and a first circuit. The first circuit includes a transistorand a capacitor. The first circuit has a function of sensing that apotential retained in the capacitor is lower than a reference potential,a function of generating a first signal and a second signal on the basisof the sensing result, and a function of turning on the transistor inresponse to the second signal and resetting the potential retained inthe capacitor to an initialization state. It is possible to startrefresh of the memory cell in response to the first signal.

In this specification and the like, ordinal numbers such as first,second, and third are used to avoid confusion among components, and theterms do not limit the components numerically or do not limit the order.

Note that in this specification and the like, a semiconductor devicerefers to a device that utilizes semiconductor characteristics, andmeans a circuit including a semiconductor element (e.g., a transistor ora diode), a device including the circuit, and the like. Thesemiconductor device also means any device that can function byutilizing semiconductor characteristics. For example, an integratedcircuit, and a chip including an integrated circuit are allsemiconductor devices. Moreover, a memory device, a display device, alight-emitting device, a lighting device, an electronic appliance, andthe like themselves might be semiconductor devices, or might eachinclude a semiconductor device.

A transistor includes three nodes (terminals) called a gate, a source,and a drain. A gate is a node that controls the conduction state of atransistor. Depending on the conductivity type of the transistor orlevels of potentials applied to the nodes (terminals), one of a pair ofnodes (an input node and an output node) functions as a source and theother functions as a drain. In general, in an n-channel transistor, anode to which a low potential is applied is referred to as a source, anda node to which a high potential is applied is referred to as a drain.In contrast, in a p-channel transistor, a node to which a low potentialis applied is referred to as a drain, and a node to which a highpotential is applied is referred to as a source. In this specification,the two terminals other than the gate may be referred to as a firstterminal and a second terminal.

In this specification and the like, to clarify a circuit configurationand circuit operation, one of two nodes (an input node and an outputnode) of a transistor is fixed as a source and the other is fixed as adrain in some cases. It is needless to say that, depending on a drivingmethod, the magnitude relationship between potentials applied to threeterminals of the transistor might be changed, and the source and thedrain might be interchanged. Thus, in one embodiment of the presentinvention, the distinction between the source and drain of thetransistor is not limited to that described in this specification andthe drawings.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can be determined that one embodiment of the invention in which a connection portionis not specified is disclosed in this specification and the like, insome cases. In particular, in the case where the number of portions towhich the terminal is connected might be plural, it is not necessary tospecify the portions to which the terminal is connected. Therefore, itmight be possible to constitute one embodiment of the invention byspecifying only portions to which some of terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected.

One embodiment of the present invention can provide a novelsemiconductor device and a method for driving the novel semiconductordevice. One embodiment of the present invention can provide asemiconductor device with improved reliability and a method for drivingthe semiconductor device. One embodiment of the present invention canprovide a memory device capable of reducing power consumption and amethod for driving the memory device. One embodiment of the presentinvention can provide a memory device capable of optimizing a refreshcycle and a method for driving the memory device. One embodiment of thepresent invention can provide a memory device capable of performingtemperature compensation and a method for driving the memory device.

Note that the description of these effects does not disturb theexistence of other effects. In one embodiment of the present invention,there is no need to achieve all the effects described above. In oneembodiment of the present invention, an object other than the aboveobjects, an effect other than the above effects, and a novel featurewill be apparent from the description of the specification and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration example of amemory device.

FIGS. 2A to 2D are circuit diagrams each illustrating a configurationexample of a memory cell.

FIGS. 3A and 3B are circuit diagrams each illustrating a configurationexample of a memory cell array.

FIG. 4 is a block diagram illustrating a configuration example of amonitor circuit.

FIG. 5A to 5C are circuit diagrams illustrating configuration examplesof a monitor circuit, a comparator, and a writing circuit, respectively.

FIG. 6 is a circuit diagram illustrating a configuration example of amonitor circuit.

FIG. 7 is a block diagram illustrating a configuration example of amemory cell array.

FIG. 8 is a block diagram illustrating a structure example of a bank.

FIG. 9 is a block diagram illustrating a configuration example of a mat.

FIG. 10 is a circuit diagram illustrating a configuration example of acircuit having a function of generating a signal str.

FIGS. 11A to 11D illustrate a structure example of an OS transistor:FIG. 11A is a top view; FIG. 11B is a cross-sectional view taken along aline y1-y2; FIG. 11C is a cross-sectional view taken along a line x1-x2;and FIG. 11D is a cross-sectional view taken along a line x3-x4.

FIGS. 12A to 12D illustrate a structure example of an OS transistor:FIG. 12A is a top view; FIG. 12B is a cross-sectional view taken along aline y1-y2; FIG. 12C is a cross-sectional view taken along a line x1-x2;and FIG. 12D is a cross-sectional view taken along a line x3-x4.

FIGS. 13A to 13D illustrate a structure example of an OS transistor:FIG. 13A is a top view; FIG. 13B is a cross-sectional view taken along aline y1-y2; FIG. 13C is a cross-sectional view taken along a line x1-x2;and FIG. 13D is a cross-sectional view taken along a line x3-x4.

FIGS. 14A to 14D illustrate a structure example of an OS transistor:FIG. 14A is a top view; FIG. 14B is a cross-sectional view taken along aline y1-y2; FIG. 14C is a cross-sectional view taken along a line x1-x2;and FIG. 14D is a cross-sectional view taken along a line x3-x4.

FIGS. 15A to 15D illustrate a structure example of an OS transistor:FIG. 15A is a top view; FIG. 15B is a cross-sectional view taken along aline y1-y2; FIG. 15C is a cross-sectional view taken along a line x1-x2;and FIG. 15D is a cross-sectional view taken along a line x3-x4.

FIGS. 16A to 16D illustrate a structure example of an OS transistor:FIG. 16A is a top view; FIG. 16B is a cross-sectional view taken along aline y1-y2; FIG. 16C is a cross-sectional view taken along a line x1-x2;and FIG. 16D is a cross-sectional view taken along a line x3-x4.

FIG. 17A is an enlarged view of a portion in FIG. 12B, and FIG. 17B isan energy band diagram of an OS transistor.

FIG. 18 is cross-sectional views illustrating an example of the devicestructure of a memory device.

FIG. 19 is a block diagram illustrating an example of a processing unit(CPU).

FIG. 20 is a block diagram illustrating an example of a processing unit(RFID tag).

FIGS. 21A to 21F illustrate usage examples of an RFID tag.

FIG. 22A is a flow chart showing an example of a method formanufacturing an electronic component, and FIG. 22B is a schematicperspective view illustrating a structure example of an electroniccomponent.

FIGS. 23A to 23H each illustrate an example of an electronic appliance.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described.Note that the present invention is not limited to the followingdescription. It will be readily appreciated by those skilled in the artthat modes and details of the present invention can be modified invarious ways without departing from the spirit and scope of the presentinvention. The present invention therefore should not be construed asbeing limited to the following description of the embodiments.

In the drawings, the same components, components having similarfunctions, components formed of the same material, or components formedat the same time are denoted by the same reference numerals in somecases, and description thereof is not repeated in some cases. When thesame reference numerals need to be distinguished from each other, “_1”,“_2”, “[n]”, “[m, n]”, or the like may be added to the referencenumerals. For example, in the case where a plurality of wirings WLW in amemory cell array are individually distinguished from each other, thewiring WLW in the second row may be described as a wiring WLW[2] usingan address number (row number) of the memory cell array.

In this specification, the clock signal CK is abbreviated to “a signalCK”, “CK”, or the like in some cases. The same applies to othercomponents (e.g., signal, voltage, potential, circuit, element,electrode, and wiring).

Embodiments of the present invention are described below, and any of theembodiments can be combined as appropriate. In addition, in the casewhere some structure examples are given in one embodiment, any of thestructure examples can be combined as appropriate.

Embodiment 1

In this embodiment, a memory device is described as an example of asemiconductor device.

□□ Configuration Example of Memory Device□□

FIG. 1 is a block diagram illustrating a configuration example of amemory device. A memory device 10 in FIG. 1 can be used as a dynamicrandom access memory (DRAM).

The memory device 10 includes a memory cell array 20, a row addressbuffer 31, a column address buffer 32, a row decoder 33, a columndecoder 34, a row driver circuit 35, a column driver circuit 36, aninput/output circuit 37, a control logic circuit 40, a clock generationcircuit 41, a selection circuit 42, a monitor circuit 50, and a refreshcontrol circuit 60.

Signals are input to the memory device 10 from the outside. Examples ofthe signals input from the outside are a clock signal CK, commandsignals (/CS, /RAS, /CAS, and /WE), an address signal ADDR, and a datasignal WDATA. The data signal WDATA is a write data signal. The commandsignal /CS is a chip select signal. The command signal /RAS is a rowaddress strobe signal. The command signal /CAS is a column addressstrobe signal. The command signal /WE is a write enable signal. To drivethe memory device 10, a high power supply potential VDD and a low powersupply potential VSS are input from the outside. For example, thepotential VSS can be a ground potential or 0 V.

The control logic circuit 40 has a function of controlling the wholememory device 10. The control logic circuit 40 has a function ofdecoding the command signals (/CS, /RAS, /CAS, and /WE) input from theoutside. The command signals processed by the control logic circuit 40are not limited to these signals, and another command signal can beinput depending on the circuit configuration and operations of thememory device 10. Furthermore, when one or more of the command signalsare unnecessary, these signals are not necessarily input. The controllogic circuit 40 generates control signals for circuits included in thememory device 10 on the basis of decoded command data, command datastored in the control logic circuit 40, or the like, and outputs them tothe respective circuits.

The clock generation circuit 41 has a function of generating, from anexternal clock signal CK, an internal clock signal to be used in thememory device 10. Furthermore, the clock generation circuit 41 has afunction of controlling supply of the internal clock signal.

The refresh control circuit 60 has at least a function of generating arow address signal radd2. The row address signal radd2 is generated inan address generation circuit 61 included in the refresh control circuit60. The row address signal radd2 has a function of specifying a row inwhich a refresh operation is to be carried out. The refresh controlcircuit 60 may further have a function of controlling refresh operationsof the memory cell array 20. The refresh control circuit 60 generates acontrol signal for executing a refresh operation and the row addresssignal radd2 on the basis of a signal rfsh from the monitor circuit 50,the control signal from the control logic circuit 40, the command signalinput from the outside, and the like. Furthermore, the control logiccircuit 40 may generate all or some of the control signals for executingrefresh operations.

The input/output circuit 37 has a function of controlling input/outputof the data signal WDATA and a data signal RDATA to/from the memorydevice 10, a function of controlling writing of the data signal WDATA tothe memory cell array 20, a function of controlling reading of data fromthe memory cell array 20, a function of generating the data signal RDATAfrom data read from the memory cell array 20 and outputting the datasignal RDATA, and the like. The data signal RDATA is a digital signal.

The address signal ADDR is input to the row address buffer 31 and thecolumn address buffer 32. The row address buffer 31 has a function ofretaining a row address signal. The selection circuit 42 has a functionof selecting one of a row address signal radd1 output from the rowaddress buffer 31 and the row address signal radd2 output from therefresh control circuit 60 and outputting the selected signal.

The row decoder 33 has a function of decoding a row address signal radd0input from the selection circuit 42. The row driver circuit 35 has afunction of generating a signal which is to be output to a wiring WLW.The wiring WLW in a row specified by the row address signal radd0 isselected by the row driver circuit 35.

The column decoder 34 has a function of decoding a column address signalcadd output from the column address buffer 32. The column driver circuit36 includes a reading circuit and a writing circuit. The reading circuithas a function of reading data from the memory cells 21 in a columnspecified by the column address signal cadd. The reading circuitincludes a sense amplifier (also referred to as a main amplifier) andhas a function of detecting and amplifying a potential differencebetween a pair of wirings BLW. Specifically, the sense amplifier has afunction of amplifying a potential difference between a referencevoltage (e.g., VDD/2) and the wiring BLW. The potential amplified by thesense amplifier is output to the input/output circuit 37. The writingcircuit has a function of outputting a data signal to the wiring BLW ina column specified by the column address signal cadd. The writingcircuit may include a precharge circuit having a function of prechargingthe wiring BLW.

<Memory Cell, Memory Cell Array>

FIGS. 2A to 2D are circuit diagrams each illustrating a configurationexample of a memory cell, and FIGS. 3A and 3B are circuit diagrams eachillustrating a configuration example of a memory cell array. Note thatthe conductivity types of transistors in FIGS. 2A to 2D can be changedas appropriate. For example, although a transistor MA1 in FIG. 2A is ap-channel transistor, it can be an n-channel transistor. The memory cell21 may be a binary memory cell capable of retaining 1-bit data, or maybe a multilevel memory cell capable of retaining data of 2 or more bits.

A memory cell 121 in FIG. 2A includes a node FN1, a transistor MW1, thetransistor MA1, and a capacitor C1. The memory cell 121 is a gain cellwhich includes two transistors. The node FN1 functions as a potentialretention portion that retains a potential corresponding to data. Thetransistor MW1 can function as a writing transistor. The transistor MW1is turned on, so that a data signal is written to the node FN1. Thetransistor MA1 can function as a reading transistor. Furthermore, thetransistor MA1 can function as an amplifying transistor that amplifiesthe potential retained in the node FN1.

The capacitor C1 can function as a storage capacitor of the node FN1.Note that load capacitance of the node FN1 corresponds to combinedcapacitance of the capacitor C1, which is intentionally provided, andparasitic capacitance of the node FN1 (e.g., gate capacitance of thetransistor MA1). The load capacitance can function as storagecapacitance of the node FN1. Therefore, in some cases, it is possible toretain the potential of the node FN1 without providing the capacitor C1,depending on the device configuration and the like of the memory cell121.

FIG. 3A illustrates an example of a memory cell array formed with suchmemory cells 121. The memory cells 121 are arranged in two rows and twocolumns in FIG. 3A. Symbols such as [m] and [2n−1] which are given tothe reference numerals in FIG. 3A indicate a row address or a columnaddress. Here, m is an integer greater than or equal to 2 and n is aninteger greater than or equal to 1.

In a memory cell array 22 in FIG. 3A, the wiring WLW and a wiring WLCare provided in each row, and the wiring BLW is provided in each column.A wiring SL is shared by two adjacent columns. The potential of thewiring WLC may be controlled by the row driver circuit 35. The potentialof the wiring SL may be controlled by the column driver circuit 36. Inthe case where a constant potential (e.g., VDD or VSS) is supplied tothe wiring SL, the wiring SL may function as a potential supply line.

The wiring WLC and the node FN1 are capacitively coupled to each othervia the capacitor C1; therefore, the potential of the node FN1 can bechanged in accordance with the potential of the wiring WLC. That is, thecapacitor C1 can function as a capacitive capacitor, and the wiring WLCcan function as a signal supply line (potential supply line) forsupplying a signal (potential) to a terminal of the capacitor C1.

A memory cell 122 in FIG. 2B is a modification example of the memorycell 121. A transistor MW2 having a back gate electrode is providedinstead of the transistor MW1.

A memory cell 123 in FIG. 2C includes the node FN1, the transistor MW1,the transistor MA1, a transistor MR1, and the capacitor C1. The memorycell 123 is a gain cell including three transistors (3T gain cell). Thetransistor MR1 can function as a reading transistor.

FIG. 3B illustrates an example of a memory cell array formed with suchmemory cells 123. In a memory cell array 23 in FIG. 3B, the wiring WLWand a wiring WLR are provided in each row, and the wiring BLW and awiring BLR are provided in each column. In the example of FIG. 3B, thewiring SL functions as a potential supply line for supplying a constantpotential (e.g., VSS or VDD) and is shared by the memory cell array 23.The wiring WLC functions as a potential supply line for supplying aconstant potential (e.g., VSS or VDD) to a terminal of the capacitor C1and is shared by the memory cell array 23. In the case where thepotential of a first terminal of the transistor MA1 is changed by thedriving method of the memory cell 123, for example, the wiring SL may beprovided in each column or provided in every two columns as in FIG. 3A;in the case where the potential of a terminal of the capacitor C1 ischanged, for example, the wiring WLC may be provided in each row as inFIG. 3A.

Note that the wiring BLR may be provided as in the memory cell array 23,and the transistor MW1 and the wiring BLR may be electrically connectedto each other in the memory cell array 22.

A memory cell 124 in FIG. 2D which includes the node FN1, the transistorMW1, and the capacitor C1 can be used as the memory cell 21.

<Transistors in Memory Cell>

To reduce the frequency of refresh in the memory cell 21, it ispreferable that a drain current in an off-state (off-state current) of awriting transistor (specifically, the transistor MW1 and the transistorMW2) be as small as possible. To minimize the off-state current of atransistor, for example, the channel may be formed using a semiconductorhaving a band gap of greater than or equal to 2.5 eV and a carrierconcentration of less than or equal to 1×10¹⁴ cm⁻³. A semiconductorlayer having such characteristics is, for example, an oxidesemiconductor layer. It is extremely effective to use an OS transistoras the transistor MW1 in order to reduce the frequency of refresh of thememory cell 21. In the OS transistor, a normalized off-state current permicrometer of a channel width at a source-drain voltage of 10 V can beless than or equal to 10×10⁻²¹ A (10 zA (zeptoampere)).

In an OS transistor used in the memory device 10, an oxide semiconductor(OS) layer including a channel formation region may be formed of asingle oxide semiconductor film or two or more oxide semiconductorfilms. The oxide semiconductor film included in the OS layer ispreferably formed with an oxide semiconductor containing at least oneelement selected from In, Ga, Sn, and Zn. As such an oxide, anIn—Sn—Ga—Zn oxide, an In—Ga—Zn oxide, an In—Sn—Zn oxide, an In—Al—Znoxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Znoxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In oxide, a Sn oxide, a Zn oxide, or thelike can be used. In addition, the oxide may contain an element otherthan In, Ga, Sn, and Zn, for example, an oxide semiconductor containingSiO₂.

There is no particular limitation on the transistor MA1 and thetransistor MR1. In the case where the transistor MA1 and the transistorMR1 are n-channel transistors, they can be OS transistors like thetransistor MW1. For example, the channels of the transistors MA1 and MR1can be formed using a semiconductor of a Group 14 element (e.g., Si, C,or Ge). A typical example of such a transistor is a silicon transistor.A transistor formed using a semiconductor substrate (bulk or SOI) can beused as each of the transistor MR1 and the transistor MAL For example,the transistor MA1 can be a Si transistor formed over a bulk-type singlecrystal silicon substrate or a Si transistor formed over an SOI-typesingle crystal silicon substrate. Note that a Si transistor is atransistor whose a channel is formed of silicon.

A semiconductor substrate for manufacturing the memory device 10 is notlimited to a single crystal silicon substrate, and the semiconductorsubstrate can be, for example, a single-material semiconductor substrateof silicon, germanium, or the like or a compound semiconductor substrateof silicon carbide, silicon germanium, gallium arsenide, galliumnitride, indium phosphide, zinc oxide, gallium oxide, or the like. Thecrystallinity of the semiconductor substrate is preferably singlecrystal but not limited thereto and may be amorphous, polycrystalline,or microcrystalline.

In the case where the memory device 10 is manufactured using thesemiconductor substrate, an OS transistor can be provided to be stackedover an element region of the semiconductor substrate. When the memorydevice 10 has such a device structure, a combined memory in which thememory device 10 and a logic circuit (e.g., a CPU core) are incorporatedin the same IC chip can be obtained. This enables a reduction in thesize of the memory cell 21; therefore, an increase in the capacity ofthe memory cell array 20 can be facilitated. Note that the devicestructure of the memory device 10 is described in Embodiment 3.

<<Monitor Circuit>>

The monitor circuit 50 has a function of generating the signal rfsh forrequesting a refresh operation. Specifically, the signal rfsh has afunction of permitting execution of refresh and can function as atrigger signal for a self refresh operation. FIG. 4 illustrates aconfiguration example of the monitor circuit 50. The monitor circuit 50in FIG. 4 includes a replica circuit 51, a reading circuit 52, and awriting circuit 53.

It is difficult to directly detect a potential retained in the memorycell 21; therefore, the replica circuit 51 is provided as an alternativereference memory cell a potential of which is detected. The replicacircuit 51 includes a node MNT which retains a potential. The readingcircuit 52 has a function of reading a potential Vmnt of the node MNT inthe replica circuit 51 and generating the signal rfsh and a signal stron the basis of the potential Vmnt. The writing circuit 53 has afunction of writing data to the replica circuit 51 in response to thesignal str. That is, the monitor circuit 50 has a function of writing apotential to the replica circuit 51, a function of monitoring thepotential retained in the replica circuit 51, and a function ofgenerating the signal rfsh and the signal str on the basis of themonitoring result.

Refresh begins in response to the signal rfsh generated in the monitorcircuit 50, whereby the refresh cycle of the memory device 10 can bedynamically changed. Optimization of the refresh cycle is effective inreducing power consumption. The electrical characteristics of thetransistors provided in the memory cell 21 change depending ontemperature; therefore, the retention characteristics of the memory cell21 also change depending on the temperature. The retentioncharacteristics of the replica circuit 51 also change depending on thetemperature in a manner similar to those of the memory cell 21; thus,temperature compensation of the refresh cycle is possible. To optimizethe refresh cycle, the replica circuit 51 is preferably providedphysically close to the memory cell array 20.

Thus, refresh can be performed with appropriate timing owing to themonitor circuit 50; as a result, the power consumption can be reduced.The monitor circuit 50 enables self refresh to be executed in the memorydevice 10 with appropriate timing even without a timer function.Furthermore, even without a temperature detecting function, temperaturecompensation of the refresh cycle is possible. In addition, data can besecurely retained in each memory cell 21 for a long period, which leadsto high reliability of the memory device 10. In the case where thememory cell 21 is a multilevel memory cell, which can store multileveldata, the amount of allowable change in retention potential is smallerthan that of a binary memory cell, and thus the refresh cycle is short.The monitor circuit 50 makes it possible to inhibit an increase in powerconsumption and securely retain data for a long period even in amultilevel memory cell.

Accordingly, a reduction in the power consumption of an electroniccomponent or an electronic appliance provided with the memory device 10,an improvement in the reliability thereof, stable operation with fewmalfunctions thereof, and the like can be achieved.

Configuration Example 1 of Monitor Circuit

FIG. 5A illustrates an example of the circuit configuration of a monitorcircuit. A monitor circuit 101 in FIG. 5A includes a replica circuit111, a comparator (CMP) 112, and a circuit 113. FIG. 5B illustrates anexample of the circuit configuration of the CMP 112.

<Replica Circuit>

The replica circuit 111 includes the node MNT, a transistor Mrp1, and acapacitor Crp1. Like the transistor MW1, the transistor Mrp1 canfunction as a writing transistor. The capacitor Crp1 functions as astorage capacitor for retaining the potential Vmnt of the node MNT. Afirst terminal of the capacitor Crp1 is connected to the node MNT, and asecond terminal thereof is supplied with a constant potential (e.g., VSSor a ground potential). A gate of the transistor Mrp1 of the replicacircuit 111 is electrically connected to an output of the circuit 113, afirst terminal thereof is connected to a wiring BLrp, and a secondterminal thereof is electrically connected to the node MNT. The wiringBLrp can function as a bit line to which a potential that is to bewritten to the replica circuit 111 is input.

To optimize the refresh cycle, it is preferable that the transistor Mrp1of the replica circuit 111 be formed in the same process as the writingtransistor of the memory cell 21. Furthermore, it is preferable that thereplica circuit 111 have lower retention characteristics than the memorycell 21. Thus, refresh can be more certainly executed before data islost in the memory cell 21. The amount of leakage of charge from thedata retention portion (MNT) of the replica circuit 111 is preferablylarger than that of the memory cell 21. Specifically, at least one ofthe following and the like is employed: to increase the channel width ofthe transistor Mrp1; to reduce the capacitance of the capacitor Crp1;and to increase a reference potential Vref. For example, in the casewhere the specifications and electrical characteristics of thetransistor Mrp1 and the capacitor Crp1 are made equal to those of thememory cell 21, the reference potential Vref of the CMP 112 isincreased.

<Reading Circuit>

The CMP 112 is a circuit corresponding to the reading circuit 52 in FIG.4 and has a function of determining whether or not a potential Vnmt ofthe node MNT is lower than the reference potential Vref. The CMP 112 hasa function of comparing the reference potential Vref with the potentialVmnt and outputting the comparison result at either of two logic states,i.e., at a high level potential or a low level potential. In the exampleof FIG. 5A, an output signal of the CMP 112 is the signal rfsh, and thesignal rfsh is also used as the signal str. A circuit capable ofperforming a logical operation of the signal rfsh and another controlsignal or the like may be separately provided so as to generate thesignal str.

The CMP 112 in FIG. 5B includes an amplifier circuit 115, a circuit 116,and a transistor Mp1. A signal en is input to each gate of a transistorMn1 and the transistor Mp1. The transistor Mn1 can function as a currentsource for the amplifier circuit 115. An output of the amplifier circuit115 is a high level potential when Vmnt>Vref is satisfied, and is a lowlevel potential when Vmnt<Vref is satisfied. The circuit 116 has afunction of processing an output signal of the amplifier circuit 115 tochange the signal into a signal rfsh having a high logic level (“H”) ora low logic level (“L”). In the example of FIG. 5B, a buffer circuit isused as the circuit 116, and the circuit 116 includes an odd number ofstages of inverter circuits. The logic level of the signal rfsh is “L”when the output of the amplifier circuit 115 is a high level potential,and is “H” when the output of the amplifier circuit 115 is a low levelpotential.

The signal en is used as an enable signal that activates the CMP 112.When the signal en is at “L”, the amplifier circuit 115 does notfunction, and the output of the amplifier circuit 115 is constantly ahigh level potential owing to the transistor Mp1. Thus, in a period inwhich the signal en is at “L”, the signal rfsh is constantly at “L”, sothat the refresh control circuit 60 does not operate.

For example, while the power supply potential VDD is supplied to thememory device 10, VDD may be input as the signal en. Alternatively, thesignal en at “H” may be input to the CMP 112 at regular or irregularintervals. For example, the signal en at “H” may be input at intervalsof 1×10⁻³ times or more and 0.5 times or less a retention period of thememory device 10. The signal en at “H” may be input at the timing whenthe power supply of the memory device 10 is performed or stopped so thatthe potential Vmnt of the replica circuit 111 can be detected.

<Writing Circuit>

The circuit 113 corresponds to the writing circuit 53 in FIG. 4. In theexample of FIG. 5A, the circuit 113 includes a circuit 131 and a circuit132. The circuit 131 and the circuit 132 each include an even number ofstages of inverter circuits and can function as a delay circuit. Notethat in the case where the transistor Mrp1 is a p-channel transistor, anodd number of stages of inverter circuits may be provided in the circuit131. The circuit 131 has a function of delaying the signal str togenerate a delay signal strdly1. The circuit 132 has a function ofdelaying the signal strdly1 to generate a delay signal strdly2. Thepotential of the signal strdly1 can be set to a high level owing to ahigh power supply potential of the inverter circuit of the last stage ofthe circuit 131, and the potential of the signal strdly1 can be set to ahigh level owing to a high power supply potential of the invertercircuit of the last stage of the circuit 132.

The signal strdly1 is input to the gate of the transistor Mrp1, and thesignal strdly2 is input to the wiring BLrp. When the signal rfsh is setat “H”, the signal strdly1 and the signal strdly2 each having a highlevel potential are generated in the circuit 113 and output to thereplica circuit 111. Thus, after the gate of the transistor Mrp1 becomesat a high level potential, a potential to be written to the node MNT isinput to the first terminal of the transistor Mrp1. Therefore, in themonitor circuit 101, the node MNT of the replica circuit 111 can bereset to the initial state in advance of refresh of the memory cellarray 20. For example, the high power supply potential of the invertercircuit of the last stage of the circuit 132 is VDD, the potential ofthe node MNT in the initial state of the replica circuit 111 is VDD.

(Another Configuration Example)

Instead of the circuit 113, a circuit 114 (see FIG. 5C) not includingthe circuit 132 can be used as the writing circuit 53. In this case, thesignal strdly1 is input to the gate of the transistor Mrp1 and thewiring BLrp. The circuit 113 including the circuit 132 is preferable asthe writing circuit 53. The reasons are as follows, for example: thewiring BLrp can be set to a high level potential after transition of thetransistor Mrp1 of the replica circuit 111 from a non-conduction stateto a conduction state begins; and the high level potential of the gateof the transistor MW1 can be made different from the high levelpotential of the wiring BLrp.

The use of the monitor circuit 101 makes it possible to performrewriting of the replica circuit 111 independently of the self refreshof the memory device 10. The replica circuit 111 can be brought into theinitial state at the timing when self refresh begins; therefore, thestate of the replica circuit 111 can be regarded as equivalent to thatof the memory cell 21 on which rewriting has been performed lastly ofthe memory cell array 20. Thus, the potential Vmnt of the replicacircuit 111 is monitored, and then the signal rfsh is generated on thebasis of the monitoring result, whereby execution of excessive refreshcan be avoided while the reliability of the memory cell array 20 isensured; as a result, the power consumption of the memory device 10 canbe reduced, and the processing efficiency can be improved.

Configuration Example 2 of Monitor Circuit

FIG. 6 illustrates another configuration example of the monitor circuit.A monitor circuit 102 in FIG. 6 includes a plurality of rows of replicacircuits 111. With such a structure, the potentials of the nodes MNT canbe averaged. The circuit 132 is not necessarily provided in the monitorcircuit 102.

Another Configuration Example of Memory Device

In the memory device 10 in FIG. 1, self refresh of all the memory cells21 in the memory cell array 20 is executed in response to the signalrfsh output from the monitor circuit 50. It is possible to executerefresh of part of the memory cell array 20. For example, refresh may beperformed on a bank-by-bank basis, a mat-by-mat basis, or a row-by-rowbasis.

Configuration Example 2

The memory cell array in FIG. 1 may have a multibank structure with aplurality of banks. In the example of FIG. 1, self refresh of all thebanks is executed in response to the signal rfsh. In the case where thememory cell array 20 has a multibank structure, the monitor circuit 50and the refresh control circuit 60 can be provided in each bank, so thatself refresh of each bank can be performed independently. Such anexample is illustrated in FIG. 7. In the example of FIG. 7, the memorycell array 20 is divided into four banks (BNK0, BNK1, BNK2, and BNK3).The banks BNK0 to BNK3 are provided with monitor circuits (50_0, 50_1,50_2, and 50_3) and refresh control circuits (60_0, 60_1, 60_2, and60_3), respectively. Writing and reading operations of the banks BNK0 toBNK3 can be independently performed. Therefore, the banks BNK0 to BNK3are each provided with the row decoder 33, the column decoder 34, therow driver circuit 35, and the column driver circuit 36. A referencenumeral such as “_0” is written in each circuit in FIG. 7 to indicatethat the circuit is provided in each bank.

For example, when the signal rfsh is generated in the monitor circuit50_1, a row address signal and a control signal for refreshing the bankBNK1 are generated in the refresh control circuit 60_1 and output to aperipheral circuit corresponding to BNK1. In the banks except the bankBNK1, a refresh operation is not executed and an operation can beperformed by normal access request.

Each of the banks BNK0 to BNK3 can be divided into blocks each of whichcan independently execute reading and writing operations. The block maybe referred to as a mat. FIG. 8 illustrates an example in which the bankBNK0 is divided into blocks in eight rows and eight columns. Each of theBNK1 to BNK3 also has a similar configuration. FIG. 8 illustrates thebank BNK0 and some peripheral circuits. A mat 80 includes a sub-rowdriver circuit 75 and a sense amplifier portion 76 in addition to amemory cell array 81 included in the bank BNK0. The sub-row drivercircuit 75 is included in a row driver circuit 35_0, and the senseamplifier portion 76 is included in a column driver circuit 36_0. Themonitor circuit 50 and the refresh control circuit 60 are provided ineach mat 80. Such a configuration allows each mat 80 to be refreshedindependently. Furthermore, the configuration enables optimization ofthe refresh cycle of each mat 80. Thus, refresh can be performedefficiently in the memory device 10.

The monitor circuit 50 and the refresh control circuit 60 may beprovided in each row of the mats 80 (in each wiring WLW). FIG. 9illustrates such a configuration example. In this case, a circuitgenerating the signal str may be provided in the monitor circuit 50.FIG. 10 illustrates an example of a circuit that generates the signalstr. A circuit 150 in FIG. 10 includes an AND circuit 151 and an ORcircuit 152. A signal ME and a signal RA are input to the AND circuit151. The signals ME and RA are each a command signal which is generatedin the memory device 10. The signal ME is a mat enable signal, and thesignal RA is a refresh address signal. For example, the signal ME isgenerated by the row decoder 33 of the bank including the mats 80. Thesignal RA is generated in the row decoder of the mat 80.

Regardless of the logic levels of the signals ME and RA, in the circuit150, the logic level of the signal str is set at “H” when the signalrfsh is set at “H”, the replica circuit 51 of the monitor circuit 50 inthe corresponding row is initialized, and the memory cells 21 in the roware refreshed. Furthermore, regardless of the logic level of the signalrfsh, when the signals ME and RA are set at “H”, the logic level of thesignal str is “H”. That is, even in the case where writing or refresh inreading is executed in response to request of the signals ME and RA, thereplica circuit 51 in the corresponding row can be reset. Thus, refreshcan be executed in each row in an appropriate cycle. As a result, animprovement in the reliability and a reduction in the power consumptionof the memory device 10 can be achieved.

Embodiment 2

In this embodiment, an OS transistor is described.

<<Structure Example 1 of OS Transistor>>

FIGS. 11A to 11D illustrate an example of a structure of an OStransistor. FIG. 11A is a plan view illustrating a structure example ofan OS transistor. FIG. 11B is a cross-sectional view taken along a liney1-y2, FIG. 11C is a cross-sectional view taken along a line x1-x2, andFIG. 11D is a cross-sectional view taken along a line x3-x4. Here, insome cases, the direction of the line y1-y2 is referred to as a channellength direction, and the direction of the line x1-x2 is referred to asa channel width direction. Accordingly, FIG. 11B illustrates across-sectional structure of the OS transistor in the channel lengthdirection, and FIGS. 11C and 11D each illustrate a cross-sectionalstructure of the OS transistor in the channel width direction.

An OS transistor 501 illustrated in FIGS. 11A to 11D includes a backgate. The OS transistor 501 is formed over an insulating surface, here,over an insulating layer 511. The insulating layer 511 is formed over asurface of a substrate 510. The OS transistor 501 is covered with aninsulating layer 514 and an insulating layer 515. Note that theinsulating layers 514 and 515 may be regarded as components of the OStransistor 501. The OS transistor 501 includes an insulating layer 512,an insulating layer 513, oxide semiconductor (OS) layers 521 to 523, aconductive layer 530, a conductive layer 531, a conductive layer 541,and a conductive layer 542. Here, the OS layers 521, 522, and 523 arecollectively referred to as an OS layer 520.

The insulating layer 513 includes a region functioning as a gateinsulating layer. The conductive layer 530 functions as a gateelectrode. The conductive layer 531 functions as a back gate electrode.A constant potential, the same potential or signal supplied to theconductive layer 530, or a potential or signal that is different fromthat supplied to the conductive layer 530 may be supplied to theconductive layer 531. The conductive layer 541 and the conductive layer542 function as a source electrode and a drain electrode.

As illustrated in FIGS. 11B and 11C, the OS layer 520 includes a regionwhere the OS layer 521, the OS layer 522, and the OS layer 523 arestacked in this order. The insulating layer 513 covers this stackedregion. The conductive layer 530 overlaps the stacked region with theinsulating layer 513 positioned therebetween. The conductive layer 541and the conductive layer 542 are provided over the stacked film formedof the OS layer 521 and the OS layer 522 and are in contact with a topsurface of this stacked film and a side surface positioned in thechannel length direction of the stacked film. In the example of FIGS.11A to 11D, the conductive layers 541 and 542 are also in contact withthe insulating layer 512. The OS layer 523 is formed to cover the OSlayers 521 and 522 and the conductive layers 541 and 542. A bottomsurface of the OS layer 523 is in contact with a top surface of the OSlayer 522.

The conductive layer 530 is formed so as to surround, in the channelwidth direction, the region where the OS layers 521 to 523 are stackedin the OS layer 520 with the insulating layer 513 positionedtherebetween (see FIG. 11C). Therefore, a gate electric field in thevertical direction and a gate electric field in the lateral directionare applied to this stacked region. In the OS transistor 501, “the gateelectric field” refers to an electric field generated by a voltageapplied to the conductive layer 531 (gate electrode layer). Accordingly,the whole stacked region of the OS layers 521 to 523 can be electricallysurrounded by the gate electric fields, so that a channel is formed inthe whole OS layer 522 (bulk), in some cases. Thus, high on-statecurrent characteristics of the OS transistor 501 can be achieved.

In this specification, a structure of a transistor in which asemiconductor is electrically surrounded by a gate electric field as inthe above transistor is referred to as “a surrounded channel (s-channel)structure”. The OS transistor 501 has the s-channel structure. With thiss-channel structure, a large amount of current can flow between thesource and the drain of the transistor, so that a high drain current inan on state (high on-state current) can be achieved.

By employing the s-channel structure in the OS transistor 501, channelformation region controllability by a gate electric field applied to theside surface of the OS layer 522 becomes easy. In the structure wherethe conductive layer 530 reaches below the OS layer 522 and faces theside surface of the OS layer 521, higher controllability can beachieved, which is preferable. Consequently, the subthreshold swing (Svalue) of the OS transistor 501 can be made small, so that ashort-channel effect can be reduced. Thus, the s-channel structure isappropriate for miniaturization.

When an OS transistor which has a three-dimensional structure as in theOS transistor 501 illustrated in FIGS. 11A to 11D, the channel lengthcan be less than 100 nm. By the miniaturization, the circuit area of theOS transistor can be made small. The channel length of the OS transistoris preferably less than 65 nm, further preferably less than or equal to30 nm or less than or equal to 20 nm.

A conductor functioning as a gate of a transistor, a conductorfunctioning as a source of a transistor, and a conductor functioning asa drain of a transistor are referred to as a gate electrode, a sourceelectrode, and a drain electrode, respectively. A region functioning asa source of a transistor and a region functioning as a drain of atransistor are referred to as a source region and a drain region,respectively. In this specification, a gate electrode might be referredto as a gate, a drain electrode or a drain region might be referred toas a drain, and a source electrode or a source region might be referredto as a source.

The channel length refers to, for example, a distance between a sourceand a drain in a region where a semiconductor (or a portion where acurrent flows in a semiconductor when a transistor is on) and a gateoverlap each other or a region where a channel is formed in a plan viewof the transistor. In one transistor, channel lengths in all regions arenot necessarily the same. In other words, the channel length of onetransistor is not fixed to one value in some cases. Therefore, in thisspecification, the channel length is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where a current flows in a semiconductor when a transistoris on) and a gate overlap with each other, or a region where a channelis formed. In one transistor, channel widths in all regions are notnecessarily the same. In other words, the channel width of onetransistor is not fixed to one value in some cases. Therefore, in thisspecification, a channel width is any one of values, the maximum value,the minimum value, or the average value in a region where a channel isformed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in aplan view of a transistor (hereinafter referred to as an apparentchannel width) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a plan view of the transistor, andits influence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of the semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the plan view.

In this specification, in the case where the term “channel width” issimply used, it may denote an apparent channel width in some cases.Alternatively, in this specification, in the case where the term“channel width” is simply used, it may denote an effective channel widthin some cases. Note that the values of a channel length, a channelwidth, an effective channel width, an apparent channel width, asurrounded channel width, and the like can be determined by obtainingand analyzing a cross-sectional TEM image and the like.

<<Structure Example 2 of OS Transistor>>

An OS transistor 502 illustrated in FIGS. 12A to 12D is a modificationexample of the OS transistor 501. FIG. 12A is a plan view of the OStransistor 502. FIG. 12B is a cross-sectional view taken along a liney1-y2, FIG. 12C is a cross-sectional view taken along a line x1-x2, andFIG. 12D is a cross-sectional view taken along a line x3-x4. Note thatto clarify the device structure, FIG. 12A does not illustrate somecomponents.

Like the OS transistor 501, the OS transistor 502 illustrated in FIGS.12A to 12D also has the s-channel structure. The OS transistor 502 doesnot include the conductive layer 531 and is different from the OStransistor 501 in the shapes of the conductive layer 541 and theconductive layer 542. The conductive layer 541 and the conductive layer542 in the OS transistor 502 are formed from a hard mask used forforming the stacked film of the OS layer 521 and the OS layer 522.Therefore, the conductive layer 541 and the conductive layer 542 are notin contact with the side surfaces of the OS layer 521 and the OS layer522 (FIG. 12D).

Through the following steps, the OS layers 521 and 522 and theconductive layers 541 and 542 can be formed. A two-layer oxidesemiconductor film including the OS layers 521 and 522 is formed. Asingle-layer or stacked-layer conductive film is formed over the oxidesemiconductor film. This conductive film is etched, so that a hard maskis formed. Using this hard mask, the two-layer oxide semiconductor filmis etched to form the OS layers 521 and 522. Then, the hard mask isetched to form the conductive layer 541 and the conductive layer 542.

<Structure Examples 3 and 4 of OS Transistor>

An OS transistor 503 illustrated in FIGS. 13A to 13D is a modificationexample of the OS transistor 501, and an OS transistor 504 illustratedin FIGS. 14A to 14D is a modification example of the OS transistor 502.In each of the OS transistors 503 and 504, the OS layer 523 and theinsulating layer 513 are etched using the conductive layer 530 as amask. Thus, an edge of the OS layer 523 and an edge of the insulatinglayer 513 are substantially aligned with an edge of the conductive layer530.

<Structure Examples 5 and 6 of OS Transistor>

An OS transistor 505 illustrated in FIGS. 15A to 15D is a modificationexample of the OS transistor 501, and an OS transistor 506 illustratedin FIGS. 16A to 16D is a modification example of the OS transistor 502.The OS transistors 505 and 506 each include a layer 551 between the OSlayer 523 and the conductive layer 541 and a layer 552 between the OSlayer 523 and the conductive layer 542.

The layers 551 and 552 can each be formed using a layer of a transparentconductor, an oxide semiconductor, a nitride semiconductor, or anoxynitride semiconductor, for example. The layers 551 and 552 can eachbe formed using an n-type oxide semiconductor layer or a conductivelayer which has a higher resistance than the conductive layers 541 and542. The layers 551 and 552 may be formed using, for example, a layercontaining indium, tin, and oxygen, a layer containing indium and zinc,a layer containing indium, tungsten, and zinc, a layer containing tinand zinc, a layer containing zinc and gallium, a layer containing zincand aluminum, a layer containing zinc and fluorine, a layer containingzinc and boron, a layer containing tin and antimony, a layer containingtin and fluorine, a layer containing titanium and niobium, or the like.Any of these layers may contain one or more of hydrogen, carbon,nitrogen, silicon, germanium, and argon.

The layers 551 and 552 may have a property of transmitting visiblelight. Alternatively, the layers 551 and 552 may have a property of nottransmitting visible light, ultraviolet light, infrared light, or X-raysby reflecting or absorbing it. In some cases, such a property cansuppress a change in electrical characteristics of the transistor due tostray light.

As each of the layers 551 and 552, a layer which does not form aSchottky barrier with the OS layer 523 is preferably used. Thus,on-state characteristics of the OS transistors 505 and 506 can beimproved.

The layers 551 and 552 are each preferably a layer that has a higherresistance than the conductive layers 541 and 542. The layers 551 and552 each preferably have resistance lower than the channel resistance ofthe transistor. For example, the layers 551 and 552 may have aresistivity higher than or equal to 0.1 Ωcm and lower than or equal to100 Ωcm, higher than or equal to 0.5 Ωcm and lower than or equal to 50Ωcm, or higher than or equal to 1 Ωcm and lower than or equal to 10 Ωcm.The layers 551 and 552 having a resistivity within the above range canreduce electric field concentration in a boundary portion between thechannel and the drain. Therefore, a change in electrical characteristicsof the transistor can be suppressed. In addition, a punch-throughcurrent generated by an electric field from the drain can be reduced.Thus, a transistor with a small channel length can have favorablesaturation characteristics. Note that in a circuit configuration wherethe source and the drain do not interchange, only one of the layers 551and 552 (e.g., the layer on the drain side) may preferably be provided.

The components of the OS transistors 501 and 502 are described below.

<Oxide Semiconductor Layer>

As the semiconductor material of the OS layers 521 to 523, typically, anIn—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M is Ga, Y, Zr, La,Ce, or Nd) is used. In addition, the OS layers 521 to 523 are notlimited to the oxide layers containing indium. The OS layers 521 to 523can be a Zn—Sn oxide layer, a Ga—Sn oxide layer, a Zn—Mg oxide layer,for example. The OS layer 522 is preferably formed using an In-M-Znoxide. Each of the OS layers 521 and 523 is preferably formed using a Gaoxide.

A case where the OS layers 521 to 523 are formed using In-M-Zn oxidefilms formed by a sputtering method is described. The atomic ratio ofmetal elements of a target for the deposition of an In-M-Zn oxide thatis used for forming the OS layer 522 is In:M:Zn=x₁:y₁:z₁. The atomicratio of metal elements of a target that is used for forming the OSlayer 521 and the OS layer 523 is In:M:Zn=x₂:y₂:z₂.

For forming the OS layer 522, a polycrystalline target of an In-M-Znoxide in which x₁/y₁ is greater than or equal to ⅓ and less than orequal to 6, or greater than or equal to 1 and less than or equal to 6,and z₁/y₁ is greater than or equal to ⅓ and less than or equal to 6, orgreater than or equal to 1 and less than or equal to 6 is preferablyused. Note that when z₁/y₁ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film is easily formed. Typical examples of theatomic ratio of the metal elements in the target include In:M:Zn=1:1:1,In:M:Zn=1:1:1.2, In:M:Zn=2:1:1.5, In:M:Zn=2:1:2.3, In:M:Zn=2:1:3,In:M:Zn=3:1:2, and In:M:Zn=4:2:4.1. Note that the CAAC-OS is an oxidesemiconductor including a c-axis aligned crystal part, and is describedlater. It is preferable that the CAAC-OS film have no spinel crystalstructure in particular. Thus, the reliability and electricalcharacteristics of the transistor including the CAAC-OS film can beimproved.

In the target used for forming the OS layers 521 and 523, x₂/y₂ ispreferably less than x₁/y₁, and z₂/y₂ is preferably greater than orequal to ⅓ and less than or equal to 6, further preferably greater thanor equal to 1 and less than or equal to 6. Note that when z₂/y₂ isgreater than or equal to 1 and less than or equal to 6, a CAAC-OS filmis easily formed. Typical examples of the atomic ratio of the metalelements in the target include In:M:Zn=1:3:2, In:M:Zn 1:3:4,In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4, In:M:Zn=1:4:5,In:M:Zn=1:4:6, In:M:Zn=1:4:7, In:M:Zn=1:4:8, In:M:Zn=1:5:5,In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8, and In:M:Zn=1:6:8.

In the In-M-Zn oxide film, the proportions of atoms in the atomic ratiovary within a range of ±40% as an error. For example, the atomic ratioof metal elements contained in an oxide semiconductor film depositedusing an oxide target of In:M:Zn=4:2:4.1 is approximately In:M:Zn=4:2:3.

<Energy Band Structure>

Next, a function and an effect of the OS layer 520 in which the OSlayers 521, 522, and 523 are stacked are described using an energy banddiagram in FIG. 17B. FIG. 17A is an enlarged view of a channel region ofthe OS transistor 502 illustrated in FIG. 12B. FIG. 17B shows an energyband diagram of a portion taken along a dotted line z1-z2 (the channelformation region of the OS transistor 502) in FIG. 17A. The OStransistor 502 is described below as an example, but the same can applyto the OS transistor 501 and the OS transistors 503 to 506.

In FIG. 17B, Ec512, Ec521, Ec522, Ec523, and Ec513 indicate the energyof the conduction band minimum of the insulating layer 512, the OS layer521, the OS layer 522, the OS layer 523, and the insulating layer 513,respectively.

Here, a difference in energy between the vacuum level and the conductionband minimum (the difference is also referred to as “electron affinity”)corresponds to a value obtained by subtracting an energy gap from adifference in energy between the vacuum level and the valence bandmaximum (the difference is also referred to as an ionization potential).Note that the energy gap can be measured using a spectroscopicellipsometer (UT-300 manufactured by HORIBA JOBIN YVON S.A.S.). Theenergy difference between the vacuum level and the valence band maximumcan be measured using an ultraviolet photoelectron spectroscopy (UPS)device (VersaProbe manufactured by ULVAC-PHI, Inc.).

Note that an In—Ga—Zn oxide which is formed using a sputtering targethaving an atomic ratio of In:Ga:Zn=1:3:2 has an energy gap ofapproximately 3.5 eV and an electron affinity of approximately 4.5 eV.An In—Ga—Zn oxide which is formed using a sputtering target having anatomic ratio of In:Ga:Zn=1:3:4 has an energy gap of approximately 3.4 eVand an electron affinity of approximately 4.5 eV. An In—Ga—Zn oxidewhich is formed using a sputtering target having an atomic ratio ofIn:Ga:Zn=1:3:6 has an energy gap of approximately 3.3 eV and an electronaffinity of approximately 4.5 eV. An In—Ga—Zn oxide which is formedusing a sputtering target having an atomic ratio of In:Ga:Zn=1:6:2 hasan energy gap of approximately 3.9 eV and an electron affinity ofapproximately 4.3 eV. An In—Ga—Zn oxide which is formed using asputtering target having an atomic ratio of In:Ga:Zn=1:6:8 has an energygap of approximately 3.5 eV and an electron affinity of approximately4.4 eV. An In—Ga—Zn oxide which is formed using a sputtering targethaving an atomic ratio of In:Ga:Zn=1:6:10 has an energy gap ofapproximately 3.5 eV and an electron affinity of approximately 4.5 eV.An In—Ga—Zn oxide which is formed using a sputtering target having anatomic ratio of In:Ga:Zn=1:1:1 has an energy gap of approximately 3.2 eVand an electron affinity of approximately 4.7 eV. An In—Ga—Zn oxidewhich is formed using a sputtering target having an atomic ratio ofIn:Ga:Zn=3:1:2 has an energy gap of approximately 2.8 eV and an electronaffinity of approximately 5.0 eV.

Since the insulating layer 512 and the insulating layer 513 areinsulators, Ec512 and Ec513 are closer to the vacuum level than Ec521,Ec522, and Ec523 (i.e., the insulating layer 512 and the insulatinglayer 513 have a smaller electron affinity than the OS layers 521, 522,and 523).

Ec521 is closer to the vacuum level than Ec522. Specifically, Ec521 ispreferably located closer to the vacuum level than Ec522 by 0.05 eV ormore, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV orless, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

Ec523 is closer to the vacuum level than Ec522. Specifically, Ec523 ispreferably located closer to the vacuum level than Ec522 by 0.05 eV ormore, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV orless, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

Mixed regions are formed in the vicinity of the interface between the OSlayer 521 and the OS layer 522 and the interface between the OS layer522 and the OS layer 523; thus, the energy at the bottom of theconduction band changes continuously. In other words, no state or fewstates exist at these interfaces.

Accordingly, electrons transfer mainly through the OS layer 522 in thestacked-layer structure having the above energy band structure.Therefore, even if an interface state exists at the interface betweenthe OS layer 521 and the insulating layer 512 or the interface betweenthe OS layer 523 and the insulating layer 513, the interface statehardly influences the transfer of electrons. In addition, since nointerface state or few interface states exist at the interface betweenthe OS layer 521 and the OS layer 522 and the interface between the OSlayer 523 and the OS layer 522, the transfer of electrons is notinterrupted in the region. Consequently, the transistor 502 includingthe above stacked oxide semiconductors can have high field-effectmobility.

Although trap states Et502 due to impurities or defects might be formedin the vicinity of the interface between the OS layer 521 and theinsulating layer 512 and the interface between the OS layer 523 and theinsulating layer 513 as illustrated in FIG. 17B, the OS layer 522 can beseparated from the trap states owing to the existence of the OS layers521 and 523.

In the transistor 502, in the channel width direction, the top surfaceand side surfaces of the OS layer 522 are in contact with the OS layer523, and the bottom surface of the OS layer 522 is in contact with theOS layer 521 (see FIG. 12C). Surrounding the OS layer 522 by the OSlayers 521 and 523 in this manner can further reduce the influence ofthe trap states.

However, when the energy difference between Ec522 and Ec521 or Ec523 issmall, an electron in the OS layer 522 might reach the trap state bypassing over the energy difference. Since the electron is trapped at thetrap state, a negative fixed charge is generated at the interface withthe insulating film, causing the threshold voltage of the transistor tobe shifted in the positive direction.

Therefore, each of the energy gaps between Ec521 and Ec522 and betweenEc522 and Ec523 is preferably 0.1 eV or more, or further preferably 0.15eV or more, in which case a change in the threshold voltage of thetransistor can be reduced and the transistor can have favorableelectrical characteristics.

The band gap of each of the OS layers 521 and 523 is preferably widerthan that of the OS layer 522.

For the OS layers 521 and 523, a material containing Ga Y, Zr, La, Ce,or Nd with a higher atomic ratio than that used for the OS layer 522 canbe used, for example. Specifically, any of the above metal elements withan atomic ratio 1.5 times or more, preferably 2 times or more, orfurther preferably 3 times or more as high as the metal element in theOS layer 522 is contained. Any of the above metal elements is stronglybonded to oxygen and thus has a function of suppressing generation of anoxygen vacancy in the oxide semiconductor. That is, an oxygen vacancy isless likely to be generated in the OS layers 521 and the 523 than in theOS layer 522.

When the OS layers 521, 522, and 523 are In-M-Zn oxides containing atleast indium, zinc, and M (M is Ga, Y, Zr, La, Ce, or Nd) and the atomicratio of In to M and Zn of the OS layer 521 is x₁:y₁:z₁, that of the OSlayer 522 is x₂:y₂:z₂, and that of the OS layer 523 is x₃:y₃:z₃, y₁/x₁and y₃/x₃ are preferably larger than y₂/x₂. Furthermore, y₁/x₁ and y₃/x₃are 1.5 times or more as large as y₂/x₂, preferably 2 times or more aslarge as y₂/x₂, or further preferably 3 times or more as large as y₂/x₂.In this case, the transistor can have stable electrical characteristicswhen y₂ is greater than or equal to x₂ in the OS layer 522. However,when y₂ is three times or more as large as x₂, the field-effect mobilityof the transistor is reduced; accordingly, y₂ is preferably smaller than3 times x₂.

An In-M-Zn oxide film satisfying the above conditions can be formedusing an In-M-Zn oxide target satisfying the above atomic ratio of metalelements.

In the case where Zn and O are not taken into consideration, theproportion of In and the proportion of M in the OS layer 521 and the OSlayer 523 are preferably less than 50 atomic % and greater than 50atomic %, respectively, or further preferably less than 25 atomic % andgreater than 75 atomic %, respectively. In the case where Zn and O arenot taken into consideration, the proportion of In and the proportion ofM in the OS layer 522 are preferably greater than 25 atomic % and lessthan 75 atomic %, respectively, or further preferably greater than 34atomic % and less than 66 atomic %, respectively.

Furthermore, at least one of the OS layers 521 and 523 does notnecessarily contain indium in some cases. For example, the OS layer 521and/or the OS layer 523 can be formed using a gallium oxide film.

The thickness of each of the OS layers 521 and 523 is greater than orequal to 3 nm and less than or equal to 100 nm, or preferably greaterthan or equal to 3 nm and less than or equal to 50 nm. The thickness ofthe OS layer 522 is greater than or equal to 3 nm and less than or equalto 200 nm, preferably greater than or equal to 3 nm and less than orequal to 100 nm, or further preferably greater than or equal to 3 nm andless than or equal to 50 nm. The OS layer 523 is preferably thinner thanthe OS layers 521 and 522.

Note that in order that an OS transistor in which a channel is formed inan oxide semiconductor have stable electrical characteristics, it iseffective to make the oxide semiconductor intrinsic or substantiallyintrinsic by reducing the concentration of impurities in the oxidesemiconductor. The term “substantially intrinsic” refers to the statewhere an oxide semiconductor has a carrier density lower than1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, or further preferablylower than 1×10¹³/cm³.

In the oxide semiconductor, hydrogen, nitrogen, carbon, silicon, and ametal element other than a main component are impurities. For example,hydrogen and nitrogen form donor levels to increase the carrier density,and silicon forms impurity levels in the oxide semiconductor. Theimpurity level becomes a trap, which might deteriorate the electricalcharacteristics of the transistor. Therefore, it is preferable to reducethe concentration of the impurities in the OS layers 521, 522, and 523and at interfaces between the OS layers.

In order to make the oxide semiconductor intrinsic or substantiallyintrinsic, for example, the concentration of silicon at a certain depthof the oxide semiconductor or in a region of the oxide semiconductor,which is measured by SIMS, is lower than 1×10¹⁹ atoms/cm³, preferablylower than 5×10¹⁸ atoms/cm³, or further preferably lower than 1×10¹⁸atoms/cm³. The concentration of hydrogen at a certain depth of the oxidesemiconductor or in a certain region of the oxide semiconductor is lowerthan or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹atoms/cm³, or still further preferably lower than or equal to 5×10¹⁸atoms/cm³. The concentration of nitrogen at a certain depth of the oxidesemiconductor or in a certain region of the oxide semiconductor is lowerthan 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³,or still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In addition, in the case where the oxide semiconductor includes acrystal, the crystallinity of the oxide semiconductor might be decreasedif silicon or carbon is included at high concentration. In order not tolower the crystallinity of the oxide semiconductor, for example, theconcentration of silicon at a certain depth of the oxide semiconductoror in a certain region of the oxide semiconductor is lower than 1×10¹⁹atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, or further preferablylower than 1×10¹⁸ atoms/cm³. Furthermore, the concentration of carbon ata certain depth of the oxide semiconductor or in a certain region of theoxide semiconductor is lower than 1×10¹⁹ atoms/cm³, preferably lowerthan 5×10¹⁸ atoms/cm³, or further preferably lower than 1×10¹⁸atoms/cm³, for example.

A transistor in which a highly purified oxide semiconductor is used fora channel formation region as described above has an extremely lowoff-state current. In the case where the voltage between a source and adrain is set at approximately 0.1 V, 5 V, or 10 V, for example, theoff-state current standardized on the channel width of the transistorcan be as low as several yoctoamperes per micrometer to severalzeptoamperes per micrometer.

<Crystal Structure of Oxide Semiconductor Film>

A structure of an oxide semiconductor film that forms the OS layer 520is described.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. The term “perpendicular” indicates that the angleformed between two straight lines is greater than or equal to 80° andless than or equal to 100°, and accordingly also includes the case wherethe angle is greater than or equal to 85° and less than or equal to 95°.The term “substantially perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to □ □° and lessthan or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

(CAAC-OS Film)

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSfilm, which is obtained using a transmission electron microscope (TEM),a plurality of crystal parts can be observed. However, in thehigh-resolution TEM image, a boundary between crystal parts, that is, agrain boundary is not clearly observed. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is less likelyto occur.

In the high-resolution cross-sectional TEM image of the CAAC-OS filmobserved in a direction substantially parallel to the sample surface,metal atoms arranged in a layered manner are seen in the crystal parts.Each metal atom layer has a configuration reflecting unevenness of asurface over which the CAAC-OS film is formed (hereinafter, the surfaceis referred to as a formation surface) or a top surface of the CAAC-OSfilm, and is arranged parallel to the formation surface or the topsurface of the CAAC-OS film.

While in the high-resolution planar TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface, metal atoms arranged in a triangular or hexagonal configurationare seen in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that in structural analysis of the CAAC-OS film including anInGaZnO₄ crystal by an out-of-plane method, another peak may appear when2θ is around 36°, in addition to the peak at 2θ of around 31°. The peakof 2θ at around 36° indicates that a crystal having no c-axis alignmentis included in part of the CAAC-OS film. It is preferable that in theCAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ notappear at around 36°.

The CAAC-OS film is an oxide semiconductor film with a low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. An element (specifically, silicon or the like)having higher strength of bonding to oxygen than a metal elementincluded in an oxide semiconductor film extracts oxygen from the oxidesemiconductor film, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor film. A heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (or molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and decreases crystallinity.Additionally, the impurity contained in the oxide semiconductor filmmight serve as a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. For example, oxygen vacancies in the oxide semiconductorfilm serve as carrier traps or serve as carrier generation sources whenhydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus has a low carrier density in some cases. Thus, atransistor including the oxide semiconductor film rarely has a negativethreshold voltage (is rarely normally on). The highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasfew carrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. An electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released. Thetrapped electric charge may behave like a fixed electric charge. Thus,the transistor which includes the oxide semiconductor film having a highimpurity concentration and a high density of defect states might haveunstable electrical characteristics.

In an OS transistor using the CAAC-OS film, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small.

(Microcrystalline Oxide Semiconductor Film)

A microcrystalline oxide semiconductor film has a region in which acrystal part is observed and a region in which a crystal part is notobserved clearly in a high-resolution TEM image. In most cases, acrystal part in the microcrystalline oxide semiconductor film is greaterthan or equal to 1 nm and less than or equal to 100 nm, or greater thanor equal to 1 nm and less than or equal to 10 nm. A microcrystal with asize greater than or equal to 1 nm and less than or equal to 10 nm, or asize greater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as nanocrystal (nc). An oxide semiconductorfilm including nanocrystal is referred to as an nc-OS (nanocrystallineoxide semiconductor) film. In a high-resolution TEM image of the nc-OS,for example, a grain boundary is not clearly observed in some cases.

In the nc-OS film, a microscopic region (e.g., a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is analyzed by an out-of-plane methodusing an X-ray beam having a diameter larger than the size of a crystalpart, a peak which shows a crystal plane does not appear. Furthermore, adiffraction pattern like a halo pattern is observed when the nc-OS filmis subjected to electron diffraction using an electron beam with a probediameter (e.g., 50 nm or larger) that is larger than the size of acrystal part (the electron diffraction is also referred to asselected-area electron diffraction). Meanwhile, spots appear in ananobeam electron diffraction pattern of the nc-OS film when an electronbeam having a probe diameter close to or smaller than the size of acrystal part is applied. Moreover, in a nanobeam electron diffractionpattern of the nc-OS film, regions with high luminance in a circular(ring) pattern are shown in some cases. Also in a nanobeam electrondiffraction pattern of the nc-OS film, a plurality of spots are shown ina ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared with an amorphous oxide semiconductor film. Therefore, thenc-OS film is likely to have a lower density of defect states than anamorphous oxide semiconductor film. Note that there is no regularity ofcrystal orientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film.

(Amorphous Oxide Semiconductor Film)

The amorphous oxide semiconductor film is an oxide semiconductor filmhaving disordered atomic arrangement and no crystal part. For example,the amorphous oxide semiconductor film does not have a specific state asin quartz.

In a high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found. When the amorphous oxidesemiconductor film is subjected to structural analysis by anout-of-plane method with an XRD apparatus, a peak which shows a crystalplane does not appear. A halo pattern is observed when the amorphousoxide semiconductor film is subjected to electron diffraction.Furthermore, a spot is not observed and a halo pattern appears when theamorphous oxide semiconductor film is subjected to nanobeam electrondiffraction.

An oxide semiconductor film may have a structure having physicalproperties intermediate between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beobserved. Furthermore, in the high-resolution TEM image, there are aregion where a crystal part is clearly observed and a region where acrystal part is not observed. In this manner, growth of the crystal partoccurs due to the crystallization of the a-like OS film, which isinduced by a slight amount of electron beam employed in the TEMobservation. In contrast, crystallization by a slight amount of electronbeam used for TEM observation is less observed in the nc-OS film havinggood quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers of three In—O layers and six Ga—Zn—Olayers are layered in the c-axis direction. Accordingly, the spacingbetween these adjacent layers is equivalent to the lattice spacing onthe (009) plane (also referred to as d value). The value is calculatedto 0.29 nm from crystal structure analysis. Thus, each of the latticefringes having a distance therebetween of from 0.28 nm to 0.30 nm isregarded as corresponding to the a-b plane of the InGaZnO₄ crystal,focusing on the lattice fringes in the high-resolution TEM image.

The film density of the oxide semiconductor film varies depending on thestructure in some cases. For example, the structure of an oxidesemiconductor film can be estimated by comparing the film density of theoxide semiconductor film with the film density of a single crystal oxidesemiconductor film having the same composition as the oxidesemiconductor film. For example, the film density of the a-like OS filmis higher than or equal to 78.6% and lower than 92.3% of the filmdensity of the single crystal oxide semiconductor film having the samecomposition. For example, the film density of the nc-OS film and theCAAC-OS film is higher than or equal to 92.3% or and lower than 100% ofthe film density of the single crystal oxide semiconductor film havingthe same composition. Note that it is difficult to form an oxidesemiconductor film having a film density of lower than 78% of the filmdensity of the single crystal oxide semiconductor film having the samecomposition.

Specific examples of the above description are given. For example, inthe case of an oxide semiconductor film having an atomic ratio ofIn:Ga:Zn=1:1:1, the film density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor film having an atomic ratio ofIn:Ga:Zn=1:1:1, the film density of the a-like OS film is higher than orequal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case ofthe oxide semiconductor film having an atomic ratio of In:Ga:Zn=1:1:1,the film density of each of the nc-OS film and the CAAC-OS film ishigher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor film havinga certain composition cannot exist in a single crystal structure. Inthat case, single crystal oxide semiconductor films with differentcompositions are combined in an adequate ratio to calculate the densityequivalent to that of a single crystal oxide semiconductor film with thedesired composition. The film density of the single crystal oxidesemiconductor film having the desired composition can be calculatedusing a weighted average according to the combination ratio of thesingle crystal oxide semiconductor films with different compositions.Note that it is preferable to combine as few kinds of single crystaloxide semiconductor films as possible for film density calculation.

Note that an oxide semiconductor film may be a stacked film includingtwo or more of an amorphous oxide semiconductor film, an a-like OS film,a microcrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

<Substrate>

The substrate 510 is not limited to a simple supporting substrate andmay be a substrate where a device such as a transistor is formed. Inthat case, one of the conductive layers 530, 541, and 542 of the OStransistor 501 may be electrically connected to the device.

<Base Insulating Film>

The insulating layer 511 has a function of preventing impurity diffusionfrom the substrate 510. The insulating layer 512 preferably has afunction of supplying oxygen to the OS layer 520. For this reason, theinsulating layer 512 is preferably an insulating film containing oxygen,further preferably, an insulating film containing oxygen in which theoxygen content is higher than that in the stoichiometric composition.For example, a film from which oxygen molecules at more than or equal to1.0×10¹⁸ molecules/cm³ are released in thermal desorption spectroscopy(TDS) at a surface temperature of the film of higher than or equal to100° C. and lower than or equal to 700° C., or higher than or equal to100° C. and lower than or equal to 500° C. can be used. When thesubstrate 510 is a substrate where a device is formed as describedabove, the insulating layer 511 is preferably subjected to planarizationtreatment such as chemical mechanical polishing (CMP) treatment so as tohave a flat surface.

The insulating layers 511 and 512 can be formed using an insulatingmaterial of aluminum oxide, aluminum oxynitride, magnesium oxide,silicon oxide, silicon oxynitride, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide,hafnium oxide, tantalum oxide, silicon nitride, silicon nitride oxide,aluminum nitride oxide, or the like, or a mixed material of thesematerials. In this specification, oxynitride refers to a material whichincludes more oxygen than nitrogen, and nitride oxide refers to asubstance which includes more nitrogen than oxygen.

<Gate Electrode>

The conductive layer 530 is preferably formed using a metal such ascopper (Cu), tungsten (W), molybdenum (Mo), gold (Au), aluminum (Al),manganese (Mn), titanium (Ti), tantalum (Ta), nickel (Ni), chromium(Cr), lead (Pb), tin (Sn), iron (Fe), cobalt (Co), ruthenium (Ru),iridium (Ir), strontium (Sr), and platinum (Pt); an alloy mainlycontaining any of these materials; or a compound mainly containing anyof these materials.

The conductive layer 530 may have a single-layer structure or astacked-layer structure of two or more layers. For example, any of thefollowing structures can be employed: a single-layer structure of analuminum film containing silicon; a two-layer structure in which atitanium film is stacked over an aluminum film; a two-layer structure inwhich a titanium film is stacked over a titanium nitride film; atwo-layer structure in which a tungsten film is stacked over a titaniumnitride film; a two-layer structure in which a tungsten film is stackedover a tantalum nitride film or a tungsten nitride film; a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order; a single-layer structure of a Cu—Mnalloy film; a two-layer structure in which a Cu film is stacked over aCu—Mn alloy film; and a three-layer structure in which a Cu—Mn alloyfilm, a Cu film, and a Cu—Mn alloy film are stacked in this order. ACu—Mn alloy film is preferably used because of its low electricalresistance and because it forms manganese oxide at the interface with aninsulating film containing oxygen and manganese oxide can prevent Cudiffusion.

The conductive layer 530 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to have a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

<Gate Insulating Layer>

The insulating layer 513 is formed using an insulating film having asingle-layer structure or a stacked-layer structure. The insulatinglayer 513 can be formed using an insulating film containing at least oneof aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide,hafnium oxide, and tantalum oxide. The insulating layer 513 may be astack including any of the above materials. The insulating layer 513 maycontain lanthanum (La), nitrogen, zirconium (Zr), or the like as animpurity. The insulating layer 511 can be formed in a manner similar tothat of the insulating layer 513. The insulating layer 513 containsoxygen, nitrogen, silicon, hafnium, or the like, for example.Specifically, the insulating layer 513 preferably includes hafniumoxide, and silicon oxide or silicon oxynitride.

Hafnium oxide has a higher dielectric constant than silicon oxide andsilicon oxynitride. Therefore, the physical thickness can be made largeas compared with silicon oxide; thus, leakage current due to tunnelcurrent can be low. That is, a transistor with a low off-state currentcan be provided. Moreover, hafnium oxide with a crystal structure has ahigher dielectric constant than hafnium oxide with an amorphousstructure. Therefore, it is preferable to use hafnium oxide with acrystal structure in order to provide a transistor with a low off-statecurrent. Examples of the crystal structure include a monoclinic crystalstructure and a cubic crystal structure. Note that one embodiment of thepresent invention is not limited to the above examples.

<Source Electrode, Drain Electrode, Back Gate Electrode>

The conductive layers 541 and 542 and the conductive layer 531 can beformed in a manner similar to that of the conductive layer 530. A Cu—Mnalloy film is preferably used for the conductive layers 541 and 542because of its low electrical resistance and because it forms manganeseoxide at the interface with the OS layer 520 and manganese oxide canprevent Cu diffusion.

<Protective Insulating Film>

The insulating layer 514 preferably has a function of blocking oxygen,hydrogen, water, an alkali metal, an alkaline earth metal, and the like.The provision of the insulating layer 514 can prevent outward diffusionof oxygen from the OS layer 520 and entry of hydrogen, water, or thelike into the OS layer 520 from the outside. The insulating layer 514can be a nitride insulating film, for example. The nitride insulatingfilm is formed using silicon nitride, silicon nitride oxide, aluminumnitride, aluminum nitride oxide, or the like. Note that instead of thenitride insulating film having a blocking effect against oxygen,hydrogen, water, an alkali metal, an alkaline earth metal, and the like,an oxide insulating film having a blocking effect against oxygen,hydrogen, water, and the like may be provided. As the oxide insulatingfilm having a blocking effect against oxygen, hydrogen, water, and thelike, an aluminum oxide film, an aluminum oxynitride film, a galliumoxide film, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, and a hafnium oxynitride film canbe given.

An aluminum oxide film is preferably used as the insulating layer 514because it is highly effective in preventing transmission of both oxygenand impurities such as hydrogen and moisture. Thus, during and after themanufacturing process of the transistor, the aluminum oxide film cansuitably function as a protective film that has effects of preventingentry of impurities such as hydrogen and moisture, which causevariations in the electrical characteristics of the transistor, into theOS layer 520, preventing release of oxygen, which is the main componentof the OS layer 520, from the oxide semiconductor, and preventingunnecessary release of oxygen from the insulating layer 512. Inaddition, oxygen contained in the aluminum oxide film can be diffusedinto the oxide semiconductor.

<Interlayer Insulating Film>

The insulating layer 515 is preferably formed over the insulating layer514. The insulating layer 515 can be formed using an insulating filmwith a single-layer structure or a layered structure. The insulatinglayer can be formed using an insulating film containing one or more ofmagnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide.

<<Deposition Method>>

A sputtering method and a plasma-enhanced chemical vapor depositionmethod are typical examples of a method of forming an insulating film, aconductive film, a semiconductor film, and the like. The insulatingfilm, the conductive film, the semiconductor film, and the like may beformed by another method, for example, a thermal CVD method. A metalorganic chemical vapor deposition (MOCVD) method or an atomic layerdeposition (ALD) method can be employed as a thermal CVD method, forexample.

A thermal CVD method does not generate plasma and thus has an advantagethat no defect due to plasma damage is caused. Deposition by a thermalCVD method may be performed in such a manner that the pressure in achamber is set to an atmospheric pressure or a reduced pressure, and asource gas and an oxidizer are supplied to the chamber at the same timeand react with each other in the vicinity of the substrate or over thesubstrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). In such a case, a first source gas is introduced,an inert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at the same time, the inert gas serves as a carrier gas, andthe inert gas may also be introduced at the same time as theintroduction of the second source gas. Alternatively, the first sourcegas may be exhausted by vacuum evacuation instead of the introduction ofthe inert gas, and then the second source gas may be introduced. Thefirst source gas is adsorbed on the surface of the substrate to form afirst single-atomic layer; then the second source gas is introduced toreact with the first single-atomic layer; as a result, a secondsingle-atomic layer is stacked over the first single-atomic layer, sothat a thin film is formed. The sequence of the gas introduction isrepeated more than once until desired thickness is obtained, so that athin film with excellent step coverage can be formed. The thickness ofthe thin film can be adjusted by the number of repetition times of thesequence of the gas introduction; therefore, an ALD method makes itpossible to accurately adjust thickness and thus is suitable formanufacturing a minute FET.

The conductive film and the semiconductor film that are described in theabove embodiment can be formed by a thermal CVD method such as a MOCVDmethod or an ALD method. For example, in the case where an InGaZnO_(X)(X>0) film is foHned, trimethylindium, trimethylgallium, and diethylzincare used. Note that the chemical formula of trimethylindium is (CH₃)₃In.The chemical formula of trimethylgallium is (CH₃)₃Ga. The chemicalformula of dimethylzinc is (CH₃)₂Zn. Without limitation to the abovecombination, triethylgallium (chemical formula: (C₂H₅)₃Ga) can be usedinstead of trimethylgallium and dimethylzinc (chemical formula:(CH₃)₂Zn) can be used instead of diethylzinc.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced more than once to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at the same time, sothat a tungsten film is formed. Note that an SiH₄ gas may be usedinstead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, for example,an InGaZnO_(X) (X>0) film is formed using a deposition apparatusemploying ALD, an (CH₃)₃In gas and an O₃ gas are sequentially introducedmore than once to form an InO₂ layer, a (CH₃)₃Ga gas and an O₃ gas areintroduced at the same time to form a GaO layer, and then a (CH₃)₂Zn gasand an O₃ gas are introduced at the same time to form a ZnO layer. Notethat the order of these layers is not limited to this example. A mixedcompound layer such as an InGaO₂ layer, an InZnO₂ layer, a GaInO layer,a ZnInO layer, or a GaZnO layer may be formed by mixing these gases.Note that although an H₂O gas that is obtained by bubbling with an inertgas such as Ar may be used instead of an O₃ gas, it is preferable to usean O₃ gas, which does not contain H. Instead of an (CH₃)₃In gas, an(C₂H₅)₃In gas may be used. Instead of a (CH₃)₃Ga gas, a (C₂H₅)₃Ga gasmay be used. Furthermore, a (CH₃)₂Zn gas may be used.

Embodiment 3

In this embodiment, a semiconductor device structure is described. Asdescribed in Embodiment 1, a semiconductor device can be formed with aSi transistor and an OS transistor. The semiconductor device with such astructure can be reduced in size by stacking a Si transistor and an OStransistor. A structure example of the semiconductor device with such astacked-layer structure is described with reference to FIG. 18.

The device structure of the memory device 10 is described as an exampleof the semiconductor device. Typically, the memory cell 122 (FIG. 2B) isused as an example for describing the device structure of the memorydevice 10. FIG. 18 is a cross-sectional view illustrating a structureexample of the memory cell 122. In FIG. 18, a section a1-a2 shows across-sectional structure of the transistor MW1 and a transistor MA2 inthe channel length direction, and a section c1-c2 shows across-sectional structure of the transistor MW2 in the channel widthdirection.

In FIG. 18, regions where reference numerals and hatching patterns arenot given show regions formed using an insulator. The region can beformed using an insulator containing one or more of aluminum oxide,aluminum nitride oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, and tantalum oxide. Alternatively, inthese regions, an organic resin such as a polyimide resin, a polyamideresin, an acrylic resin, a siloxane resin, an epoxy resin, or a phenolresin can be used.

<Transistor MA1>

Here, a planar-type field-effect transistor is used as the transistorMA1. The transistor MA1 is manufactured using an SOI semiconductorsubstrate including a single crystal silicon layer. A substrate 400 is asubstrate (e.g., a single crystal silicon substrate) that supports thesingle crystal silicon layer. An insulating layer 401 is a buried oxidelayer (BOX layer) for insulating the single crystal silicon layer fromthe substrate 400. Needless to say, a Si transistor such as thetransistor MA1 can be manufactured using a bulk-type single crystalsilicon substrate. Moreover, the device structure of the transistor MA1is not limited to the example of FIG. 18. For example, a 3D transistorformed utilizing an projection portion of a semiconductor substrate(e.g., a fin-type transistor or a Tri-gate type transistor) can beemployed.

The transistor MA1 includes a Si layer 410, a gate insulating layer 416,and a conductor 420. In the Si layer 410, impurity regions 411, 412,413, and 414 and a channel formation region 415 are formed. The impurityregions 411 and 412 each function as a source region or a drain region.The impurity regions 413 and 414 each function as a lightly doped drain(LDD) region or an extension region. Here, the conductivity type of eachof the impurity regions 411 to 414 is an p-type. The impurity region 412has a region functioning as a wiring SL. The conductor 420 has a regionfunctioning as a gate electrode of the transistor MAL Insulating layers418 and 419 are formed on the side surfaces of the conductor 420. Theimpurity regions 411 to 414 can be formed in the Si layer 410 in aself-aligned manner by the formation of the insulating layers 418 and419. The transistor MA1 is covered with an insulating layer 402.

<Transistor MW2>

The transistor MW2 has a device structure similar to that of the OStransistor 504 including a back gate. The device structure of thetransistor MW2 is not limited thereto.

The transistor MW2 is formed over an insulating layer 403. Thetransistor MW2 includes an OS layer including a channel formationregion, a conductor 435, a conductor 436, conductors 437 and 438, and agate insulating layer 439. The transistor MW2 is covered with aninsulating layer 404 and an insulating layer 405. The OS layer 430 ofthe transistor MW2 has a three-layer structure of OS layers 431 to 433,which is similar to a three-layer structure of the OS transistor 502(FIGS. 12A to 12D). The conductor 435 has a region functioning as a backgate electrode of the transistor MW2 and the wiring BGL. The conductor436 has a region functioning as a gate electrode of the transistor MW2and the wiring WWL. The conductors 437 and 438 each function as a sourceelectrode or a drain electrode of the transistor MW2.

The insulating layer 403 serving as a base insulating layer of thetransistor MW2 is preferably formed using an insulator having a functionof preventing diffusion of hydrogen from a lower layer to the OS layer430. This has an effect of improving reliability of the Si transistor byterminating dangling bonds of silicon in the Si layer by hydrogen. Incontrast, as described above, hydrogen serves as an impurity thatreduces reliability of the OS layer in the OS transistor. Thus, theinsulating layer 403 confines hydrogen in the lower layer and diffuseshydrogen from the lower layer to an upper layer, so that reliability ofboth the transistor MA1 (Si transistor) and the transistor MW2 (OStransistor) can be improved. The insulating layer 403 can be formedusing aluminum oxide, aluminum oxynitride, gallium oxide, galliumoxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafniumoxynitride, or yttria-stabilized zirconia (YSZ), for example. Inparticular, an aluminum oxide film is preferably used because thealuminum oxide film has a high shielding (blocking) effect of preventingtransmission of both oxygen and impurities such as hydrogen andmoisture.

A conductor 450 has a region functioning as a wiring BL. The conductor450 is electrically connected to the conductor 437 of the transistor MW2through conductors 451 to 454. The conductor 450 is also electricallyconnected to the impurity region 411 of the transistor MA1 through theconductors 451 to 454 and conductors 455 and 456.

<Capacitor C1>

A region in which a conductor 461 and a conductor 462 overlap each otherwith a dielectric provided therebetween functions as the capacitor C1.The conductor 461 has a region functioning as a wiring RWL. Theconductor 462 is electrically connected to the gate electrode (theconductor 420) of the transistor MA1 through conductors 463 to 466.

FIG. 18 indicates that the circuits included in the memory device 10 canbe formed through a process for forming the transistor MA1. Thus,according to one embodiment of the present invention, a combined memory(combined RAM) which is capable of writing data at low voltage and whichhas reduced frequency of refresh can be provided.

Embodiment 4

In this embodiment, a memory device as an example of a semiconductordevice and a processing unit that processes data stored in the memorydevice are described.

<<CPU>>

FIG. 19 illustrates a CPU configuration example. A CPU 300 illustratedin FIG. 19 includes a CPU core 301, a power management unit 321, and aperipheral circuit 322. The power management unit 321 includes a powercontroller 302 and a power switch 303. The peripheral circuit 322includes a cache 304 including cache memory, a bus interface (BUS I/F)305, and a debug interface (Debug I/F) 306. The CPU core 301 includes adata bus 323, a control unit 307, a program counter (PC) 308, a pipelineregister 309, a pipeline register 310, an arithmetic logic unit (ALU)311, and a register file 312. Data is transmitted between the CPU core301 and the peripheral circuit 322 such as the cache 304 via the databus 323.

The memory device of one embodiment of the present invention can be usedfor the cache 304. Consequently, high-speed operation and low powerconsumption of the cache can be achieved and thus a semiconductor devicethat operates more rapidly or a semiconductor device with low powerconsumption can be provided.

The control unit 307 has functions of decoding and executinginstructions contained in a program such as inputted applications bycontrolling the overall operations of the PC 308, the pipeline registers309 and 310, the ALU 311, the register file 312, the cache 304, the businterface 305, the debug interface 306, and the power controller 302.

The ALU 311 has a function of performing a variety of arithmeticoperations such as four arithmetic operations and logic operations. Thecache 304 has a function of temporarily storing frequently used data.The PC 308 is a register having a function of storing an address of aninstruction to be executed next. Although not illustrated in FIG. 19,the cache 304 includes a cache controller for controlling the operationof the cache memory.

The pipeline register 309 has a function of temporarily storinginstruction data. The register file 312 includes a plurality ofregisters including a general purpose register and can store data thatis read from the main memory, data obtained as a result of arithmeticoperations in the ALU 311, or the like. The pipeline register 310 has afunction of temporarily storing data used for arithmetic operationsperformed in the ALU 311, data obtained as a result of arithmeticoperations in the ALU 311, or the like.

The bus interface 305 functions as a path for data between the CPU 300and devices outside the CPU 300. The debug interface 306 functions as apath of a signal for inputting an instruction to control debugging tothe CPU 300.

The power switch 303 has a function of controlling supply of the powersupply voltage to circuits other than the power controller 302 in theCPU 300. These circuits belong to several different power domains. Thepower switch 303 controls whether the power supply voltage is suppliedto circuits in the same power domain. The power controller 302 has afunction of controlling the operation of the power switch 303. With sucha configuration, the CPU 300 can perform power gating. An example of theflow of the power gating operation is described.

First, the CPU core 301 sets the timing for stopping the supply of thepower supply voltage in a register of the power controller 302. Next, aninstruction to start power gating is sent from the CPU core 301 to thepower controller 302. Then, the registers and the cache 304 in the CPU300 start data storing. Subsequently, the power switch 303 stops thesupply of the power supply voltage to the circuits other than the powercontroller 302 in the CPU 300. Then, an interrupt signal is input to thepower controller 302, thereby starting the supply of the power supplyvoltage to the circuits included in the CPU 300. Note that a counter maybe provided in the power controller 302 to be used to determine thetiming of starting the supply of the power supply voltage regardless ofinput of an interrupt signal. Next, the registers and the cache 304start data restoration. After that, execution of an instruction isresumed in the control unit 307.

This power gating can be performed in the entire processor or one ormore logic circuits included in the processor. The supply of power canbe stopped even for a short time. Accordingly, power consumption can bereduced at a fine granularity in space or time.

In the case where the memory device of one embodiment of the presentinvention is used in the cache 304, the cache 304 can retain data for acertain period even when the supply of a power supply voltage isstopped. Therefore, when power gating is performed, a period duringwhich data of the cache 304 is stored can be secured easily. Even whenthe supply of the power supply voltage is suddenly stopped, data in thecache 304 can be stored. In the case where data is stored outside thememory device, the time and power necessary for storing and restoringdata is required, while in the case of using the memory device of oneembodiment of the present invention, such time and power are notrequired.

<<RFID Tag>>

An RFID tag is described as an example of a processing unit. The RFIDtag is referred to as a wireless tag, an RFID, an RF tag, an ID tag, anIC tag, an IC chip, an electronic tag, a wireless IC tag, and the like.The RFID tag includes a memory circuit, stores necessary data in thememory circuit, and transmits and receives data to/from the outside byusing contactless means, for example, wireless communication. With thesefeatures, the RFID tag can be used for an individual authenticationsystem in which an object or the like is recognized by reading theindividual information, for example.

FIG. 20 is a block diagram illustrating an example of an RFID tag. AnRFID tag 800 illustrated in FIG. 20 includes an antenna 804, a rectifiercircuit 805, a constant voltage circuit 806, a demodulation circuit 807,a modulation circuit 808, a logic circuit 809, a RAM 810, a read-onlymemory (ROM) 811, and a battery 820. The memory device of one embodimentof the present invention can be used in the RAM 810. Note that decisionwhether each of these circuits is provided or not can be made asappropriate as needed. For example, although the RFID tag 800 in FIG. 20is an active type, it may be a passive type without the battery 820.

The memory device according to one embodiment of the present inventionhas a device structure capable of employing a combined memory.Therefore, in the RFID tag 800, circuits other than the antenna 804 canbe incorporated in one IC chip without complicating the manufacturingprocess. The antenna 804 whose performance corresponds to thecommunication zone is mounted on the IC chip. Note that as datatransmission methods, the following methods can be given: anelectromagnetic coupling method in which a pair of coils is provided soas to face each other and communicates with each other by mutualinduction, an electromagnetic induction method in which communication isperformed using an induction field, and a radio wave method in whichcommunication is performed using a radio wave. Any of these methods canbe used in the RFID tag 800 described in this embodiment. Although theRFID tag 800 in the example of FIG. 20 is a passive tag, it is needlessto say that the RFID tag 800 can be an active wireless tag with abuilt-in battery.

The memory device of one embodiment of the present invention can be usedfor the RAM 810. A writing data potential of the RAM 810 can be reduced;therefore, power necessary for the operation of the RFID tag 800 can bereduced, and the communication distance of the RFID tag 800 can beextended.

The antenna 804 exchanges a radio signal 803 with an antenna 802 whichis connected to a communication device 801. The rectifier circuit 805generates an input potential by rectification, for example, half-wavevoltage doubler rectification of an input alternating signal generatedby reception of a radio signal at the antenna 804 and smoothing of therectified signal with a capacitor provided in a later stage in therectifier circuit 805. Note that a limiter circuit may be provided on aninput side or an output side of the rectifier circuit 805. The limitercircuit controls electric power so that electric power which is higherthan or equal to certain electric power is not input to a circuit in alater stage if the amplitude of the input alternating signal is high andan internal generation voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Furthermore,the modulation circuit 808 performs modulation in accordance with datato be output from the antenna 804.

The logic circuit 809 decodes and processes the demodulated signal. TheRAM 810 holds the input data and includes a row decoder, a columndecoder, a memory region, and the like. Furthermore, the ROM 811 storesan identification number (ID) or the like and outputs it in accordancewith processing.

In the circuits other than the RAM 810, the OS transistors described inEmbodiment 2 can be used as n-channel transistors. Since the OStransistors have low off-state currents and high on-state currents, botha low leakage current and high-speed operation can be achieved.Furthermore, the OS transistors may be used as elements having arectifying function included in the demodulation circuit 807. Since theOS transistors have low off-state currents, the reverse currents of theelements having a rectifying function can be made low, leading toexcellent rectification efficiency. Furthermore, since the transistorsusing an oxide semiconductor can be formed through the same process,high performance of the RFID tag 800 can be achieved without an increasein process cost.

<Application Examples of RFID>

The RFID tag can be used in a wide range of fields. For example, theRFID tag 800 can be provided in objects such as bills, coins,securities, bearer bonds, documents (e.g., driver's licenses orresident's cards, see FIG. 21A), packaging containers (e.g., wrappingpaper or bottles, see FIG. 21C), recording media (e.g., DVDs or videotapes, see FIG. 21B), vehicles (e.g., bicycles, see FIG. 21D), personalbelongings (e.g., bags or glasses), foods, plants, animals, humanbodies, clothing, household goods, medical supplies such as medicine andchemicals, and electronic devices (e.g., liquid crystal display devices,EL display devices, smartphones, cellular phones, clocks, or watches),or tags on objects (see FIGS. 21E and 21F).

The RFID tag 800 is fixed to an object by being attached to a surface ofthe object or being embedded in the object. For example, the RFID tag800 is fixed to an object by being embedded in paper of a book, orembedded in an organic resin of a package. Since the RFID tag 800 can bereduced in size, thickness, and weight, it can be fixed to an objectwithout spoiling the design of the object. When the RFID tag 800 isprovided in bills, coins, securities, bearer bonds, documents, or thelike, an authentication function can be provided to the objects. The useof the authentication function can prevent forgery. Furthermore, whenthe RFID tag 800 is attached to packaging containers, recording media,personal belongings, foods, clothing, household goods, electronicdevices, or the like, a system such as an inspection system or aninventory management system can be used efficiently. When the RFID tag800 is attached to vehicles, the level of security can be raised.

A variety of kinds of information can be obtained wirelessly byincorporating a sensor unit in the RFID tag 800. The RFID tag 800including a temperature sensor circuit and/or a humidity sensor circuitcan be used for controlling temperature and/or humidity of the culturalproperties, for example.

Although the CPU and the RFID tag are described here as examples of aprocessing unit, the memory device of one embodiment of the presentinvention can be used for a variety of processing units. For example,the semiconductor memory device of one embodiment of the presentinvention can also be used for a graphics processing unit (GPU), aprogrammable logic device (PLD), a digital signal processor (DSP), amicrocontroller unit (MCU), and a custom LSI.

Embodiment 5

In this embodiment, an electronic component and electronic appliancesand the like including the electronic component are described asexamples of a semiconductor device.

<<Example of Manufacturing Method of Electronic Component>>

FIG. 22A is a flow chart showing an example of a method formanufacturing an electronic component. The electronic component is alsoreferred to as a semiconductor package or an IC package. This electroniccomponent has a plurality of standards and names depending on a terminalextraction direction and a terminal shape. Examples of the electroniccomponent are described in this embodiment.

A semiconductor device including a transistor is completed byintegrating detachable components on a printed circuit board through theassembly process (post-process). The post-process can be finishedthrough steps in FIG. 22A. Specifically, after an element substrateobtained in the wafer process is completed (Step S1), a rear surface ofthe substrate is ground (Step S2). The substrate is thinned in this stepto reduce warpage or the like of the substrate in the wafer process andto reduce the size of the electronic component itself.

The rear surface of the substrate is ground so that the substrate isdivided into a plurality of chips in a dicing process. Then, the dividedchips are separately picked up to be mounted on and bonded to a leadframe in a die bonding step (Step S3). In this die bonding step, thechip is bonded to the lead frame by an appropriate method depending on aproduct, for example, bonding with a resin or a tape. Note that in thedie bonding step, the chip may be mounted on an interposer to be bonded.

Then, wire bonding is performed to electrically connect lead of the leadframe to an electrode on the chip with a metal fine line (wire) (StepS4). A silver line or a gold line can be used as the metal fine line.Ball bonding or wedge bonding can be used as the wire bonding.

A molding step is performed to seal the wire bonded chip with an epoxyresin or the like (Step S5). With the molding step, the electronic partis filled with the resin, so that damage to a mounted circuit portion orwire due to mechanical external force can be reduced. Furthermore,deterioration in characteristics due to moisture or dust can be reduced.

Next, plate processing is performed on the lead of the lead frame. Afterthat, the lead is cut and processed (Step S6). This plate processingprevents rust of the lead and facilitates soldering at the time ofmounting the chip on a printed wiring board in a later step.

Next, printing (marking) is performed on a surface of the package (StepS7). Through the final inspection step (Step S8), the electronic part iscompleted (Step S9).

The above electronic component can include the semiconductor devicedescribed in the above embodiment. Thus, the electronic component canconsume less power and have smaller size.

FIG. 22B is a schematic perspective view of the completed electroniccomponent. FIG. 22B illustrates a schematic perspective view of a quadflat package (QFP) as an example of the electronic component. Asillustrated in FIG. 22B, an electronic component 700 includes a lead 701and a circuit portion 703. The electronic component 700 is mounted on aprinted wiring board 702, for example. When a plurality of electroniccomponents 700 are used in combination and electrically connected toeach other over the printed wiring board 702, the electronic components700 can be mounted on an electronic appliance. A completed circuit board704 is provided in the electronic appliance or the like. The electroniccomponent 700 can be used as, for example, a random access memory thatstores data or a processing unit that executes a variety of types ofprocessing, such as a microcontroller unit (MCU) or an RFID tag.

The electronic component 700 can be used as electronic component (an ICchip) of electronic appliances in a wide variety of fields, such asdigital signal processing, software-defined radio systems, avionicsystems (electronic appliances used in aircraft, such as communicationsystems, navigation systems, autopilot systems, and flight managementsystems), ASIC prototyping, medical image processing, voice recognition,encryption, bioinformatics, emulators for mechanical systems, and radiotelescopes in radio astronomy. Such an electric appliance can be usedfor display devices, personal computers (PCs), or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images). Otherexamples of the electronic appliance that can be equipped with thesemiconductor device of one embodiment of the present invention aremobile phones, game machines including portable game machines, portabledata appliances, e-book readers, cameras (e.g., video cameras anddigital still cameras), wearable display devices or terminals (e.g.,head mounted display devices, goggle-type display devices, glasses-typedisplay devices, armband display devices, bracelet-type display devices,and necklace-type display devices), navigation systems, audioreproducing devices (e.g., car audio systems and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), and vending machines. FIGS. 23A to 23H illustratespecific examples of these electronic appliances.

<<Electronic Appliance>>

FIGS. 23A to 23F illustrate structure examples of an electronicappliance which includes a display portion and is driven by a battery.

A portable game machine 900 illustrated in FIG. 23A includes a housing901, a housing 902, a display portion 903, a display portion 904, amicrophone 905, a speaker 906, an operation key 907, and the like. Thedisplay portion 903 is provided with a touch screen as an input device,which can be handled with a stylus 908 or the like.

An information terminal 910 illustrated in FIG. 23B includes a housing911, a display portion 912, a microphone 917, a speaker portion 914, acamera 913, an external connection portion 916, an operation button 915,and the like. A display panel that uses a flexible substrate and a touchscreen are provided in the display portion 912. The information terminal910 can be used as, for example, a smartphone, a mobile phone, a tabletinformation terminal, a tablet PC, or an e-book reader.

A notebook PC 920 illustrated in FIG. 23C includes a housing 921, adisplay portion 922, a keyboard 923, a pointing device 924, and thelike.

A video camera 940 illustrated in FIG. 23D includes a housing 941, ahousing 942, a display portion 943, operation keys 944, a lens 945, ajoint 946, and the like. The operation keys 944 and the lens 945 areprovided in the housing 941, and the display portion 943 is provided inthe housing 942. The housings 941 and 942 are connected to each otherwith the joint 946, and an angle between the housings 941 and 942 can bechanged with the joint 946. The direction of an image on the displayportion 943 may be changed and display and non-display of an image maybe switched depending on the angle between the housings 941 and 942.

FIG. 23E illustrates an example of a bangle-type information terminal.An information terminal 950 includes a housing 951, a display portion952, and the like. The display portion 952 is supported by the housing951 having a curved surface. A display panel formed with a flexiblesubstrate is provided in the display portion 952, whereby theinformation terminal 950 can be a user-friendly information terminalthat is flexible and lightweight.

FIG. 23F illustrates an example of a watch-type information terminal. Aninformation terminal 960 includes a housing 961, a display portion 962,a band 963, a buckle 964, an operation button 965, an input/outputterminal 966, and the like. The information terminal 960 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games.

The display surface of the display portion 962 is bent, and images canbe displayed on the bent display surface. Further, the display portion962 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, bytouching an icon 967 displayed on the display portion 962, anapplication can be started. With the operation button 965, a variety offunctions such as time setting, power ON/OFF, ON/OFF of wirelesscommunication, setting and cancellation of manner mode, and setting andcancellation of power saving mode can be performed. For example, thefunctions of the operation button 965 can be set by setting theoperating system incorporated in the information terminal 960.

The information terminal 960 can employ near field communication that isa communication method based on an existing communication standard. Inthat case, for example, mutual communication between the informationterminal 960 and a headset capable of wireless communication can beperformed, and thus hands-free calling is possible. Moreover, theinformation terminal 960 includes the input/output terminal 966, anddata can be directly transmitted to and received from anotherinformation terminal via a connector. Power charging through theinput/output terminal 966 is possible. Note that the charging operationmay be performed by wireless power feeding without using theinput/output terminal 966.

FIG. 23G illustrates an electric refrigerator-freezer as an example of ahome electric appliance. An electric refrigerator-freezer 970 includes ahousing 971, a refrigerator door 972, a freezer door 973, and the like.

FIG. 23H is an external view illustrating a structure example of a motorvehicle. A motor vehicle 980 includes a car body 981, wheels 982, adashboard 983, lights 984, and the like.

An electronic component including the semiconductor device described inthe above embodiment is provided in the electronic appliances describedin this embodiment. Thus, an electronic appliance that consumes lesspower and is capable of operating stably can be provided.

This application is based on Japanese Patent Application serial no.2014-111062 filed with Japan Patent Office on May 29, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A memory device comprising: a first wiring; asecond wiring; a memory cell; and a first circuit, wherein the memorycell comprises a first transistor and a first capacitor, wherein thefirst transistor is configured to control a conduction state between thefirst wiring and a first terminal of the first capacitor, wherein thefirst circuit comprises a second transistor and a second capacitor,wherein the second transistor is configured to control a conductionstate between the second wiring and a first terminal of the secondcapacitor, wherein the first circuit is configured to determine whethera first potential of the first terminal of the second capacitor is lowerthan a second potential, to generate a first signal and a second signalwhen the first potential is determined to be lower than the secondpotential, to turn on the second transistor in response to the secondsignal, and to apply a third potential to the second wiring in responseto the second signal, and wherein the first signal is configured tostart a refresh operation of the memory cell.
 2. The memory deviceaccording to claim 1, wherein the first circuit is configured togenerate a third signal obtained by delaying the second signal, whereinthe first circuit inputs the third signal to the second wiring, andwherein the first circuit inputs the second signal to a gate of thesecond transistor.
 3. The memory device according to claim 1, whereinthe first circuit comprises an amplifier circuit and a switch, whereinthe amplifier circuit is configured to amplify a difference between thefirst potential and the second potential, wherein the amplifier circuitcomprises a third transistor, wherein the third transistor is configureda current source, wherein the switch is configured to control aconduction state between a wiring being configured to supply a fourthpotential and the amplifier circuit, and wherein the first circuitcauses the switch to be on in a period in which the third transistor isoff.
 4. The memory device according to claim 1, wherein a channel ofeach of the first and second transistors is formed using an oxidesemiconductor.
 5. The memory device according to claim 4, wherein theoxide semiconductor comprises a crystal part with c-axis alignment. 6.The memory device according to claim 1, further comprising a thirdwiring, wherein the third wiring is electrically connected to the firsttransistor.
 7. A memory device comprising: a first wiring; a secondwiring; a memory cell; and a first circuit, wherein the memory cellcomprises a first transistor and a first capacitor, wherein the firsttransistor is configured to control a conduction state between the firstwiring and a first terminal of the first capacitor, wherein the firstcircuit comprises a second transistor and a second capacitor, whereinthe second transistor is configured to control a conduction statebetween the second wiring and a first terminal of the second capacitor,wherein the first circuit is configured to determine whether a firstpotential of the first terminal of the second capacitor is lower than asecond potential, to generate a first signal and a second signal whenthe first potential is determined to be lower than the second potential,to turn on the second transistor in response to the second signal, andto apply a third potential to the second wiring in response to thesecond signal, wherein the first signal is configured to start a refreshoperation of the memory cell, wherein the first circuit is configured togenerate a third signal obtained by delaying the second signal, whereinthe first circuit inputs the third signal to the second wiring, andwherein the first circuit inputs the second signal to a gate of thesecond transistor, wherein the first circuit comprises an amplifiercircuit and a switch, wherein the amplifier circuit is configured toamplify a difference between the first potential and the secondpotential, wherein the amplifier circuit comprises a third transistor,wherein the third transistor is configured a current source, wherein theswitch is configured to control a conduction state between a wiringbeing configured to supply a fourth potential and the amplifier circuit,and wherein the first circuit causes the switch to be on in a period inwhich the third transistor is off.
 8. The memory device according toclaim 7, wherein a channel of each of the first and second transistorsis formed using an oxide semiconductor.
 9. The memory device accordingto claim 8, wherein the oxide semiconductor comprises a crystal partwith c-axis alignment.
 10. The memory device according to claim 7,further comprising a third wiring, wherein the third wiring iselectrically connected to the first transistor.
 11. A memory devicecomprising: a memory cell array comprising: a plurality of firstwirings; a plurality of memory cells; and a plurality of first circuits,a second wiring, wherein each of the plurality of memory cells comprisesa first transistor and a first capacitor, wherein the first transistoris configured to control a conduction state between one of the pluralityof first wirings and a first terminal of the first capacitor, whereineach of the plurality of the first circuit comprises a second transistorand a second capacitor, wherein the second transistor is configured tocontrol a conduction state between the second wiring and a firstterminal of the second capacitor, wherein each of the plurality of thefirst circuit is configured to determine whether a first potential ofthe first terminal of the second capacitor is lower than a secondpotential, to generate a first signal and a second signal when the firstpotential is determined to be lower than the second potential, to turnon the second transistor in response to the second signal, and to applya third potential to the second wiring in response to the second signal,and wherein the first signal is configured to start a refresh operationof one of the plurality of memory cells.
 12. The memory device accordingto claim 11, wherein the memory cell array comprises a plurality ofbanks, wherein each of the plurality of banks comprise one of theplurality of first circuits, and wherein the first signal generated inone of the plurality of first circuits is configured to start a refreshoperation of a corresponding one of the plurality of banks.
 13. Thememory device according to claim 12, wherein each of the plurality ofbanks comprises a plurality of blocks, wherein each of the plurality ofblocks each comprises one of the plurality of the first circuit, andwherein the first signal generated in one of the plurality of firstcircuits is configured to start a refresh operation of a correspondingone of the plurality of blocks.
 14. The memory device according to claim13, wherein one of the plurality of memory cells electrically connectedto a corresponding one of the plurality of first wirings.
 15. The memorydevice according to claim 11, further comprising a plurality of thirdwirings, wherein one of the plurality of third wirings is electricallyconnected to the first transistor.